Eliminate the --enable_async flag

Eliminate the --enable_async flag

The default value for --enable_async has been true for a couple of weeks now. This
change eliminates the flag altogether, which may be a breaking change unless the
dart executable is invoked with --ignore-unrecognized-flags

R=iposva@google.com

Committed: https://code.google.com/p/dart/source/detail?r=43160

[hausner, 2015-01-26 19:37:08]

hausner@google.com changed reviewers:
+ iposva@chromium.org

[hausner, 2015-01-26 20:38:10]

Adding Devon and Keerti as FYI.

[Ivan Posva, 2015-01-26 20:38:13]

iposva@google.com changed reviewers:
+ danrubel@google.com

[Ivan Posva, 2015-01-26 20:39:36]

iposva@google.com changed reviewers:
+ iposva@google.com

[Ivan Posva, 2015-01-26 20:39:36]

LGTM once the Editor is 100% ready.

-Ivan

[hausner, 2015-01-26 23:01:42]

Committed patchset #1 (id:1) manually as r43160 (presubmit successful).