Implementation of async-await transformation on js ast.

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index 8ce30bb39f8cd0ec121860b3d122f94bb9d7de84..be603fa2904b24746741dc370cb3727263ef386d 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -84,7 +84,6 @@ class Compiler extends leg.Compiler {
             suppressWarnings: hasOption(options, '--suppress-warnings'),
             enableExperimentalMirrors:
                 hasOption(options, '--enable-experimental-mirrors'),
-            enableAsyncAwait: hasOption(options, '--enable-async'),
             generateCodeWithCompileTimeErrors:
                 hasOption(options, '--generate-code-with-compile-time-errors'),
             allowNativeExtensions:
@@ -106,10 +105,6 @@ class Compiler extends leg.Compiler {
       throw new ArgumentError("[packageRoot] must end with a /.");
     }
     if (!analyzeOnly) {
-      if (enableAsyncAwait && emitJavaScript) {
-        throw new ArgumentError(
-            "--enable-async is currently only supported with --analyze-only");
-      }
       if (allowNativeExtensions) {
         throw new ArgumentError(
             "--allow-native-extensions is only supported in combination with "