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

Review URL: https://codereview.chromium.org//876013003

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@43160 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk