Please, update Stream.contains() signature

[alsemenov]

The description for method dart:async:Stream.contains() reads:

Future contains(
Object needle
)
Checks whether needle occurs in the elements provided by this stream.

Completes the Future when the answer is known. If this stream reports an error, the Future will report that error.

I think, needle should be of type T, where T is from Stream<T>

[lrhn]

The argument type matches those of Iterable.contains and Map.containsKey and is deliberate.

We should consider whether we retain this design in Dart 2.