Support async/await in the dart2js frontend.

sdk/lib/_internal/compiler/implementation/scanner/keyword.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/keyword.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/keyword.dart b/sdk/lib/_internal/compiler/implementation/scanner/keyword.dart
index 4bf9695d4bb049cc5d256e332d8af60ede8a1f08..1e01efb608d3ed33649ddfa4674b5417888ca697 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/keyword.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/keyword.dart
@@ -67,7 +67,11 @@ class Keyword {
       const Keyword("on", isPseudo: true),
       const Keyword("show", isPseudo: true),
       const Keyword("source", isPseudo: true),
-      const Keyword("deferred", isPseudo: true)];
+      const Keyword("deferred", isPseudo: true),
+      const Keyword("async", isPseudo: true),
+      const Keyword("sync", isPseudo: true),
+      const Keyword("await", isPseudo: true),
+      const Keyword("yield", isPseudo: true)];
 
   final String syntax;
   final bool isPseudo;