Fix await/async/yield keyword recognition

Fix await/async/yield keyword recognition

Fixes issue 22870.

The VM parser didn’t distinguish between await and “await”.

Added some additional symbol recognition cleanup which should speed up parsing.

R=srdjan@google.com

Committed: https://code.google.com/p/dart/source/detail?r=44540

[hausner, 2015-03-17 17:15:14]

hausner@google.com changed reviewers:
+ srdjan@google.com

[srdjan, 2015-03-17 17:26:23]

lgtm

[hausner, 2015-03-17 17:27:04]

Committed patchset #1 (id:1) manually as r44540 (presubmit successful).