Make _SingleStreamMultiplexer accept any Stream as source.
Make _SingleStreamMultiplexer accept any Stream as source.

It's used in Stream.asBroadcast, so it should work for any
class extendsing Stream.

BUG=http://dartbug.com/8818

Review URL: https://codereview.chromium.org//12317145

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@19210 260f80e4-7a28-3924-810f-c04153c831b5

dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-lang/sdk