Update implementations of Stream.{lastWhere, firstWhere} for Dart 2 about async HOT 3 CLOSED

TypeSafeStream isn't a public type—it's used behind-the-scenes by DelegatingStream.typed() to create a casting wrapper for a given stream. But now that Stream.cast() and friends exist, there's no need for typed() at all, in DelegatingStream or anywhere else in async. Probably the best thing to do is:

• Delete all the TypeSafe* wrappers.
• Update the static typed() methods to just call cast() on their arguments.
• Mark them as deprecated and suggest users use cast() directly.
• Bump the SDK constraint to whatever version first contained cast().

from async.

#54

from async.

Landed.

from async.