Add support for `await for`.

When inside an asynchronous for loop, the translation keeps track of
the name of the current stream's controller.  It pauses and resumes
the stream around await expressions and nested asynchronous for loops.
Merge branch 'master' into await-for

Conflicts:
	lib/src/xform.dart
A more complete implementation of `await for`.

Add support for cancellation of the stream.  The stream with
subscription s is canceled as if the loop were surrounded by
`try {...} finally { await s.cancel(); }`.
Merge pull request #64 from kmillikin/await-for

Add support for `await for`.