[kernel] Support both older and newer implementations of dart:async.
[kernel] Support both older and newer implementations of dart:async.

The VM target's continuation transformer uses a class that was originally
named _StreamIteratorImpl and then renamed to _StreamIterator.  If looking
up the old name fails, try the new name.

BUG=
R=jensj@google.com

Review URL: https://codereview.chromium.org/2465163004 .

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