Because every version of extended_text depends on path_provider ^0.4.1 and xxx depends on path_provider ^0.5.0+1, extended_text is forbidden.
So, because xxx depends on extended_text ^0.2.7, version solving failed.
pub get failed (1)

import 'package:flutter/material.dart';
import 'package:flutter/gestures.dart';
import 'package:extended_text_library/extended_text_library.dart';

final double _fontSize = 16.0;
final double _fontSizeField = 18.0;

class LinkText extends SpecialText {
    static const String startKey = "https://wb.jmu.edu.cn/";
    static const String endKey = " ";

    LinkText(TextStyle textStyle, SpecialTextGestureTapCallback onTap) : super(startKey, endKey, textStyle, onTap: onTap);

    @override
    TextSpan finishText() {
        return TextSpan(
            text: " 网页链接 ",
            style: textStyle?.copyWith(color: Colors.blue, fontSize: _fontSize),
            recognizer: TapGestureRecognizer()
                ..onTap = () {
                    Map<String, dynamic> data = {'content': toString()};
                    if (onTap != null) onTap(data);
                },
        );
    }
}

class LinkOlderText extends SpecialText {
    static const String startKey = "http://wb.jmu.edu.cn/";
    static const String endKey = " ";

    LinkOlderText(TextStyle textStyle, SpecialTextGestureTapCallback onTap) : super(startKey, endKey, textStyle, onTap: onTap);

    @override
    TextSpan finishText() {
        return TextSpan(
            text: " 网页链接 ",
            style: textStyle?.copyWith(color: Colors.blue, fontSize: _fontSize),
            recognizer: TapGestureRecognizer()
                ..onTap = () {
                    Map<String, dynamic> data = {'content': toString()};
                    if (onTap != null) onTap(data);
                },
        );
    }
}

class MentionText extends SpecialText {
    static const String startKey = "<M";
    static const String endKey = "<\/M>";
    final RegExp mTagStartReg = RegExp(r"<M?\w+.*?\/?>");
    final RegExp mTagEndReg = RegExp(r"<\/M?\w+.*?\/?>");
    final int start;
    final BuilderType type;

    MentionText(TextStyle textStyle, SpecialTextGestureTapCallback onTap, {this.start, this.type})
            : super(startKey, endKey, textStyle, onTap: onTap);

    @override
    bool isEnd(String value) {
        return (getContent() + value).endsWith(endKey);
    }

    int getUidFromContent(content) {
        Iterable<Match> matches = mTagStartReg.allMatches(content);
        String result;
        for (Match m in matches) result = m.group(0);
        return int.parse(result.substring(3, result.length - 1));
    }

    String removeUidFromContent(content) {
        content = content.replaceAllMapped(mTagStartReg, (match) => "");
        content = content.replaceAllMapped(mTagEndReg, (match) => "");
        return content;
    }

    @override
    TextSpan finishText() {
        String mentionOriginalText = toString();
        String mentionText = removeUidFromContent(mentionOriginalText);
        mentionOriginalText = "${mentionOriginalText.substring(0, mentionOriginalText.length - MentionText.endKey.length)}>";

        if (type == BuilderType.extendedTextField) {
            print("mentionText: $mentionText");
            print("mentionOriginalText: $mentionOriginalText");
            return SpecialTextSpan(
                text: mentionText,
                actualText: mentionOriginalText,
                start: start,
                deleteAll: true,
                style: TextStyle(color: Colors.blue, fontSize: _fontSizeField),
            );
        } else {
            int uid = getUidFromContent(mentionOriginalText);
            return TextSpan(
                text: mentionText,
                style: TextStyle(color: Colors.blue, fontSize: _fontSize),
                recognizer: TapGestureRecognizer()
                    ..onTap = () {
                        Map<String, dynamic> data = {'content': mentionText, 'uid': uid};
                        if (onTap != null) onTap(data);
                    },
            );
        }
    }
}

class PoundText extends SpecialText {
    static const String flag = "#";
    final int start;
    final BuilderType type;
    PoundText(TextStyle textStyle, SpecialTextGestureTapCallback onTap, {this.start, this.type}) : super(flag, flag, textStyle, onTap: onTap);

    @override
    TextSpan finishText() {
        final String poundText = getContent();
        if (type == BuilderType.extendedTextField) {
            return SpecialTextSpan(
                text: "#$poundText#",
                actualText: "#$poundText#",
                start: start,
                deleteAll: true,
                style: TextStyle(color: Colors.orangeAccent, fontSize: _fontSizeField),
            );
        } else {
            return TextSpan(
                text: "#$poundText#",
                style: TextStyle(color: Colors.orangeAccent, fontSize: _fontSize),
                recognizer: TapGestureRecognizer()
                    ..onTap = () {
                        Map<String, dynamic> data = {'content': toString()};
                        if (onTap != null) onTap(data);
                    },
            );
        }
    }
}

class StackSpecialTextSpanBuilder extends SpecialTextSpanBuilder {
    final BuilderType type;
    StackSpecialTextSpanBuilder({this.type: BuilderType.extendedText});

    @override
    SpecialText createSpecialText(String flag, {TextStyle textStyle, SpecialTextGestureTapCallback onTap, int index}) {
        if (flag == null || flag == "") return null;

        if (isStart(flag, MentionText.startKey)) {
            return MentionText(textStyle, onTap, type: BuilderType.extendedText);
        } else if (isStart(flag, PoundText.flag)) {
            return PoundText(textStyle, onTap, type: BuilderType.extendedText);
        } else if (isStart(flag, LinkText.startKey)) {
            return LinkText(textStyle, onTap);
        } else if (isStart(flag, LinkOlderText.startKey)) {
            return LinkOlderText(textStyle, onTap);
        }
        return null;
    }
}

class StackSpecialTextFieldSpanBuilder extends SpecialTextSpanBuilder {
    @override
    SpecialText createSpecialText(String flag, {TextStyle textStyle, SpecialTextGestureTapCallback onTap, int index}) {
        if (flag == null || flag == "") return null;

        if (isStart(flag, MentionText.startKey)) {
            return MentionText(textStyle, onTap,
                start: index - (MentionText.startKey.length - 1), // Using minus to keep position correct.
                type: BuilderType.extendedTextField,
            );
        } else if (isStart(flag, PoundText.flag)) {
            return PoundText(textStyle, onTap, start: index, type: BuilderType.extendedTextField);
        }
        return null;
    }
}
enum BuilderType { extendedText, extendedTextField }

import 'package:flutter/material.dart';

import 'package:extended_text/extended_text.dart';

import 'package:xxx/model/SpecialText.dart';

void main() {
    runApp(TestPage());
}

class TestPage extends StatefulWidget {
    @override
    _TestPageState createState() => _TestPageState();
}

class _TestPageState extends State<TestPage> {
    bool isDetail;
    String testText = "<M 123456>@甭管是谁</M>: 天天要人！150-180元高薪充场移动兼职流量话费卡\n【充场内容】:此活动是苹果手机通讯店季度业绩冲量，需要大量人员，只需本人持身份证到现场免费领取5张移动流量卡，卡要回收，不接受勿扰！绝对安全可靠且放心（不使用三个月可以注销。没有黑名单，百度可以查），无需充值，无需交费！领完立马结20的路费，隔天过来收到多少张卡激活多少张，就按一张30来算，全程差不多10分钟就能做完，欢迎组队报名。\n【地点】：管你是哪\n【时间】：管你什么时候(任意时间段来都可以。10分钟即可）\n微信手机同号:管你什么电话号码\n🎤管你什么实习生宣讲会\n【宣讲会地点、时间】\n地点：管你什么破烂地方\n时间： 管你什么狗屁时间\n【交流群】\n请有兴趣的同学们加入海康威视&集美大学咨询QQ群：管你什么群号。\n【海康威视公司简介】\nl	“全球安防50强”第一位（a&s《安全自动化》，2016-2018）\nl	**电子科技集团公司下属最大上市企业，2010年5月上市（股票代码：002415）\nl “全球品牌500强”第216位（2018 Brand Finance榜单） \nl	**最佳雇主30强（智联招聘，2018）\nl	全球雇员超26000人\nl	业务覆盖全球150+国家及地区\nl	人工智能、云计算、大数据、物联网\n【海康威视福利】\n享受六险一金、节日津贴，生日福利，餐饮、交通、通讯、高温、医疗等补贴应有尽有，同时还有全行业有竞争力的薪酬，丰厚年终奖金，大规模股权激励与跟投，还有各色员工活动等你来体验······";

    @override
    void initState() {
        super.initState();
    }

    Widget getExtendedText(content, {isRoot}) {
        return ExtendedText(
            content != null ? "$content " : null,
            style: TextStyle(fontSize: 16.0),
            onSpecialTextTap: (dynamic data) {
                String text = data['content'];
                if (text.startsWith("#")) {
                    Scaffold.of(context).showSnackBar(
                        SnackBar(content: Text("#")),
                    );
                } else if (text.startsWith("@")) {
                    Scaffold.of(context).showSnackBar(
                        SnackBar(content: Text("@")),
                    );
                } else if (text.startsWith("http")) {
                    Scaffold.of(context).showSnackBar(
                        SnackBar(content: Text("link")),
                    );
                }
            },
            maxLines: this.isDetail ?? false ? null : 10,
            overFlowTextSpan: this.isDetail ?? false ? null : OverFlowTextSpan(
                text: "查看全文",
                style: TextStyle(
                    color: Color(0xFFE5322D),
                    fontSize: 16.0,
                ),
                background: isRoot ?? false ? Theme.of(context).canvasColor : Theme.of(context).cardColor
            ),
            specialTextSpanBuilder: StackSpecialTextSpanBuilder(),
        );
    }

    Widget card() {
        return Container(
            child: Card(
                margin: EdgeInsets.symmetric(vertical: 4.0),
                child: Stack(
                    children: <Widget>[
                        Column(
                            mainAxisSize: MainAxisSize.min,
                            children: <Widget>[
                                ListTile(
                                    leading: Container(
                                        width: 40.0,
                                        height: 40.0,
                                        child: CircleAvatar(
                                            backgroundImage: NetworkImage("http://oap99.jmu.edu.cn/face?uid=136172&size=f152"),
                                        ),
                                    ),
                                    title: Text("测试名称"),
                                    subtitle: Text("测试副标题"),
                                ),
                                Row(children: <Widget>[
                                    Expanded(
                                        child: Container(
                                            padding: const EdgeInsets.symmetric(horizontal: 16.0),
                                            child: Column(
                                                mainAxisAlignment: MainAxisAlignment.start,
                                                mainAxisSize: MainAxisSize.min,
                                                crossAxisAlignment: CrossAxisAlignment.start,
                                                children: <Widget>[
                                                    ExtendedText(
                                                        "$testText ",
                                                        style: TextStyle(fontSize: 16.0),
                                                        onSpecialTextTap: (dynamic data) {
                                                            String text = data['content'];
                                                            if (text.startsWith("#")) {
                                                                showDialog(context: context, builder: (_) => AlertDialog(
                                                                    title: Text("#"),
                                                                    content: Text(text),
                                                                ));
                                                            } else if (text.startsWith("@")) {
                                                                showDialog(context: context, builder: (_) => AlertDialog(
                                                                    title: Text("@"),
                                                                    content: Text(text),
                                                                ));
                                                            } else if (text.startsWith("http")) {
                                                                showDialog(context: context, builder: (_) => AlertDialog(
                                                                    title: Text("Link"),
                                                                    content: Text(text),
                                                                ));
                                                            }
                                                        },
                                                        maxLines: 10,
                                                        overFlowTextSpan: OverFlowTextSpan(
                                                                text: "查看全文",
                                                                style: TextStyle(
                                                                    color: Color(0xFFE5322D),
                                                                    fontSize: 16.0,
                                                                ),
                                                                background: Theme.of(context).cardColor
                                                        ),
                                                        specialTextSpanBuilder: StackSpecialTextSpanBuilder(),
                                                    )
                                                ],
                                            ),
                                        ),
                                    ),
                                ]),
                            ],
                        ),
                    ],
                ),
                elevation: 0,
                shape: RoundedRectangleBorder(borderRadius: BorderRadius.zero),
            ),
        );
    }

    @override
    Widget build(BuildContext context) {
        return MaterialApp(
            home: Scaffold(
                appBar: AppBar(
                    title: Text("反正是有问题的测试DEMO"),
                ),
                body: ListView(
                    children: <Widget>[
                        card(), card(),
                    ],
                ),
            ),
        );
    }
}

if (this.position.chatWindow > 0) {
  final youliaoSpan = ImageSpan(
    AssetImage(
        "lib/src/screens/positionGroup/resources/icon_pos_youliao.png"),
    imageWidth: 44,
    imageHeight: 19,
    fit: BoxFit.contain,
    margin: const EdgeInsets.fromLTRB(6, 2, 0, 2)
  );
  textSpanList.add(youliaoSpan);
}

if (this.position.positionSourceType == 2) {
  final netApplySpan = ImageSpan(
    AssetImage(
        "lib/src/screens/positionGroup/resources/icon_pos_netapply.png"),
    imageWidth: 26,
    imageHeight: 14,
    fit: BoxFit.contain,
    margin: const EdgeInsets.fromLTRB(6, 2, 0, 4)
  );
  textSpanList.add(netApplySpan);
}

return ExtendedText.rich(
  TextSpan(children: textSpanList),
  maxLines: 2,
  textAlign: TextAlign.left,
  overflow: ExtendedTextOverflow.ellipsis,
);

换
换
换

Test {{@extended_text}}  Custom SpecialText.

Doctor summary (to see all details, run flutter doctor -v):
[√] Flutter (Channel stable, v1.7.8+hotfix.3, on Microsoft Windows [Version 10.0.17134.950], locale zh-CN)

[√] Android toolchain - develop for Android devices (Android SDK version 28.0.3)

[√] Android Studio (version 3.4)

[√] Connected device (1 available)                                                                                                                                                                                                                                  • No issues found!

tried to set a non-finite rect.
'package:flutter/src/semantics/semantics.dart':
Failed assertion: line 1175 pos 12: 'value.isFinite'


Either the assertion indicates an error in the framework itself, or we should provide substantially more information in this error message to help you determine and fix the underlying cause.
In either case, please report this assertion by filing a bug on GitHub:
  https://github.com/flutter/flutter/issues/new?template=BUG.md

When the exception was thrown, this was the stack: 
#2      SemanticsNode.rect= (package:flutter/src/semantics/semantics.dart:1175:12)
#3      ExtendedRenderParagraph.assembleSemanticsNode (package:extended_text/src/extended_render_paragraph.dart:707:14)
...

import 'package:flutter/material.dart';
import 'package:flutter/gestures.dart';
import 'package:extended_text/extended_text.dart';
import 'package:extended_text_library/extended_text_library.dart';

class TestPage extends StatefulWidget {
  @override
  _TestPageState createState() => _TestPageState();
}

class _TestPageState extends State<TestPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: ListView(
        children: <Widget>[
          for (int i = 0; i < 10; i++)
            ExtendedText(
              """#帮帮酱#今天又是充满活力的一天
失物招领&寻物启事
①水院丢失无印良品笔盒
②招领廖锦强校园卡<M 151314>@廖锦强</M>
③寻找眼镜 汉水306
④招领航海学院徐欣的规范汉字证书<M 180321>@徐欣</M>
⑤软件1991陈昊丢失校园卡
⑥古龙大礼堂对面自行车丢失
⑦寻找校园卡19级陈雨蝶<M 198738>@陈雨蝶</M>
⑧禹州408杯子失物招领
⑨文津101捡到beats耳机""",
              style: TextStyle(fontSize: 14.0),
              onSpecialTextTap: null,
              maxLines: 8,
              overFlowTextSpan: OverFlowTextSpan(
                children: <TextSpan>[
                  TextSpan(text: " ... "),
                  TextSpan(
                    text: "全文",
                    style: TextStyle(
                      color: Colors.red,
                    ),
                  ),
                ],
              ),
              specialTextSpanBuilder: StackSpecialTextSpanBuilder(),
            ),
        ],
      ),
    );
  }
}

class MentionText extends SpecialText {
  static const String startKey = "<M";
  static const String endKey = "<\/M>";
  final RegExp mTagStartReg = RegExp(r"<M?\w+.*?\/?>");
  final RegExp mTagEndReg = RegExp(r"<\/M?\w+.*?\/?>");
  final int start;
  final BuilderType type;

  MentionText(TextStyle textStyle, SpecialTextGestureTapCallback onTap,
      {this.start, this.type})
      : super(startKey, endKey, textStyle, onTap: onTap);

  @override
  bool isEnd(String value) {
    return (getContent() + value).endsWith(endKey);
  }

  int getUidFromContent(content) {
    Iterable<Match> matches = mTagStartReg.allMatches(content);
    String result;
    for (Match m in matches) result = m.group(0);
    return int.parse(result.substring(3, result.length - 1));
  }

  String removeUidFromContent(content) {
    content = content.replaceAllMapped(mTagStartReg, (match) => "");
    content = content.replaceAllMapped(mTagEndReg, (match) => "");
    return content;
  }

  @override
  InlineSpan finishText() {
    String mentionOriginalText = toString();
    String mentionText = removeUidFromContent(mentionOriginalText);
    mentionOriginalText =
    "${mentionOriginalText.substring(0, mentionOriginalText.length - MentionText.endKey.length)}>";

    if (type == BuilderType.extendedTextField) {
      return SpecialTextSpan(
        text: mentionText,
        actualText: mentionOriginalText,
        start: start,
        deleteAll: true,
        style: textStyle?.copyWith(color: Colors.blue),
      );
    } else {
      int uid = getUidFromContent(mentionOriginalText);
      return TextSpan(
        text: mentionText,
        style: textStyle?.copyWith(color: Colors.blue),
        recognizer: TapGestureRecognizer()
          ..onTap = () {
            Map<String, dynamic> data = {'content': mentionText, 'uid': uid};
            if (onTap != null) onTap(data);
          },
      );
    }
  }
}

class PoundText extends SpecialText {
  static const String flag = "#";
  final int start;
  final BuilderType type;
  PoundText(TextStyle textStyle, SpecialTextGestureTapCallback onTap,
      {this.start, this.type})
      : super(flag, flag, textStyle, onTap: onTap);

  @override
  InlineSpan finishText() {
    final String poundText = getContent();
    if (type == BuilderType.extendedTextField) {
      return SpecialTextSpan(
        text: "#$poundText#",
        actualText: "#$poundText#",
        start: start,
        deleteAll: false,
        style: textStyle?.copyWith(color: Colors.orangeAccent),
      );
    } else {
      return TextSpan(
        text: "#$poundText#",
        style: textStyle?.copyWith(color: Colors.orangeAccent),
        recognizer: TapGestureRecognizer()
          ..onTap = () {
            Map<String, dynamic> data = {'content': toString()};
            if (onTap != null) onTap(data);
          },
      );
    }
  }
}

class StackSpecialTextSpanBuilder extends SpecialTextSpanBuilder {
  final BuilderType type;
  StackSpecialTextSpanBuilder({this.type: BuilderType.extendedText});

  @override
  SpecialText createSpecialText(String flag,
      {TextStyle textStyle, SpecialTextGestureTapCallback onTap, int index}) {
    if (flag == null || flag == "") return null;

    if (isStart(flag, MentionText.startKey)) {
      return MentionText(textStyle, onTap, type: BuilderType.extendedText);
    } else if (isStart(flag, PoundText.flag)) {
      return PoundText(textStyle, onTap, type: BuilderType.extendedText);
    }
    return null;
  }
}

class StackSpecialTextFieldSpanBuilder extends SpecialTextSpanBuilder {
  @override
  SpecialText createSpecialText(String flag,
      {TextStyle textStyle, SpecialTextGestureTapCallback onTap, int index}) {
    if (flag == null || flag == "") return null;

    if (isStart(flag, MentionText.startKey)) {
      return MentionText(
        textStyle, onTap,
        start: index -
            (MentionText.startKey.length -
                1), // Using minus to keep position correct.
        type: BuilderType.extendedTextField,
      );
    } else if (isStart(flag, PoundText.flag)) {
      return PoundText(textStyle, onTap,
          start: index, type: BuilderType.extendedTextField);
    }
    return null;
  }
}

enum BuilderType { extendedText, extendedTextField }

flutter: Another exception was thrown: NoSuchMethodError: The getter 'dx' was called on null.

[✓] Flutter (Channel beta, v1.12.13+hotfix.7-pre.1, on Mac OS X 10.15.1 19B88, locale zh-Hans-CN)