Make support for async/await syntax optional in the frontend.

pkg/compiler/lib/src/diagnostics/messages.dart

Index: pkg/compiler/lib/src/diagnostics/messages.dart
diff --git a/pkg/compiler/lib/src/diagnostics/messages.dart b/pkg/compiler/lib/src/diagnostics/messages.dart
index f442dd1598537866dc214bdcad22a303fee9b7d5..083068e7f83072f670387c9a58395cb68475e4c6 100644
--- a/pkg/compiler/lib/src/diagnostics/messages.dart
+++ b/pkg/compiler/lib/src/diagnostics/messages.dart
@@ -89,6 +89,7 @@ enum MessageKind {
   ASSIGNING_METHOD,
   ASSIGNING_METHOD_IN_SUPER,
   ASSIGNING_TYPE,
+  ASYNC_AWAIT_NOT_SUPPORTED,
   ASYNC_KEYWORD_AS_IDENTIFIER,
   ASYNC_MODIFIER_ON_ABSTRACT_METHOD,
   ASYNC_MODIFIER_ON_CONSTRUCTOR,
@@ -3239,6 +3240,11 @@ main() async* {
 """
           ]),
 
+      MessageKind.ASYNC_AWAIT_NOT_SUPPORTED:
+        const MessageTemplate(MessageKind.ASYNC_AWAIT_NOT_SUPPORTED,
+            "The async/sync* syntax is not supported on the current platform."),
+
+
       MessageKind.ASYNC_MODIFIER_ON_ABSTRACT_METHOD: const MessageTemplate(
           MessageKind.ASYNC_MODIFIER_ON_ABSTRACT_METHOD,
           "The modifier '#{modifier}' is not allowed on an abstract method.",