Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/apiimpl.dart

Index: sdk/lib/_internal/compiler/implementation/apiimpl.dart
diff --git a/sdk/lib/_internal/compiler/implementation/apiimpl.dart b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
index 5670d49a41a7433583ff7d4d4d0e36ddd6e43acf..fdabf68e5894ce7346a2b6437f0c0c1b2bb8c751 100644
--- a/sdk/lib/_internal/compiler/implementation/apiimpl.dart
+++ b/sdk/lib/_internal/compiler/implementation/apiimpl.dart
@@ -77,7 +77,8 @@ class Compiler extends leg.Compiler {
             hasIncrementalSupport:
                 forceIncrementalSupport ||
                 hasOption(options, '--incremental-support'),
-            suppressWarnings: hasOption(options, '--suppress-warnings')) {
+            suppressWarnings: hasOption(options, '--suppress-warnings'),
+            enableAsyncAwait: hasOption(options, '--enable-async')) {
     tasks.addAll([
         userHandlerTask = new leg.GenericTask('Diagnostic handler', this),
         userProviderTask = new leg.GenericTask('Input provider', this),
@@ -88,6 +89,10 @@ class Compiler extends leg.Compiler {
     if (packageRoot != null && !packageRoot.path.endsWith("/")) {
       throw new ArgumentError("packageRoot must end with a /");
     }
+    if (enableAsyncAwait && !analyzeOnly) {
+      throw new ArgumentError(
+          "--enable-async is currently only supported with --analyze-only");
+    }
   }
 
   static String extractStringOption(List<String> options,