Support async/await in the dart2js frontend.

BUG=
R=floitsch@google.com

Review URL: https://codereview.chromium.org//675113002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41485 260f80e4-7a28-3924-810f-c04153c831b5
Add test for docgen of source with async/await.

BUG=
R=floitsch@google.com

Review URL: https://codereview.chromium.org//687723007

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41486 260f80e4-7a28-3924-810f-c04153c831b5
Firefox not only slow on Windows.

Review URL: https://codereview.chromium.org//700863002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41487 260f80e4-7a28-3924-810f-c04153c831b5
Dart2js is slow when compiled in checked mode.

Review URL: https://codereview.chromium.org//698283002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41488 260f80e4-7a28-3924-810f-c04153c831b5