Update implementations of Stream.{lastWhere, firstWhere} for Dart 2

[leafpetersen]

The current transitional state of lastWhere and firstWhere from lib/src/typed/stream.dart looks like this:

   Future<T> firstWhere(bool test(T element),  {Object defaultValue(), T orElse()}) =>
       _stream.firstWhere(_validateType(test), defaultValue: defaultValue ?? orElse);

We are deprecating (and will soon remove) the defaultValue argument. So at a minimum this code needs to be updated to:

   Future<T> firstWhere(bool test(T element),  {Object defaultValue(), T orElse()}) =>
       _stream.firstWhere(_validateType(test), orElse: orElse ?? () => defaultValue());

Presumably at some point we will want to deprecate and remove defaultValue from the TypeSafeStream API as well. Presumably we will need a major version bump when we remove it, since it's an API breaking change.

@nex3 would you prefer that I do this:

• in one step
  • remove defaultValue entirely, bump the major version
• in two steps
  • remove the delegating use of defaultValue but leave it in the API marked deprecated
  • at some later point (possibly as part of a collected major release) remove defaultValue

[nex3]

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().

[leafpetersen]

dart-archive/async#54

[leafpetersen]

Landed.