Fix analyzer type checking of "await" expressions.

Fix analyzer type checking of "await" expressions.

This requires implementing the "flatten()" rule defined in the spec;
this rule also needs to be applied to static type checking of return
types in async functions.

BUG=dartbug.com/22291, dartbug.com/22293
R=brianwilkerson@google.com

Committed: https://code.google.com/p/dart/source/detail?r=43537

[Paul Berry, 2015-02-05 23:29:59]

paulberry@google.com changed reviewers:
+ brianwilkerson@google.com

[Brian Wilkerson, 2015-02-06 00:24:46]

LGTM

[Paul Berry, 2015-02-06 03:33:09]

Committed patchset #1 (id:1) manually as 43537 (presubmit successful).