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

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

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * The messages in this file should meet the following guide lines:
  *
  * 1. The message should be a complete sentence starting with an uppercase
  * letter, and ending with a period.
  *
@@ skipping 71 matching lines @@
   ADDITIONAL_ARGUMENT,
   ADDITIONAL_TYPE_ARGUMENT,
   ALREADY_INITIALIZED,
   AMBIGUOUS_LOCATION,
   AMBIGUOUS_REEXPORT,
   ASSERT_IS_GIVEN_NAMED_ARGUMENTS,
   ASSIGNING_FINAL_FIELD_IN_SUPER,
   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,
   ASYNC_MODIFIER_ON_SETTER,
   AWAIT_MEMBER_NOT_FOUND,
   AWAIT_MEMBER_NOT_FOUND_IN_CLOSURE,
   BAD_INPUT_CHARACTER,
   BEFORE_TOP_LEVEL,
   BINARY_OPERATOR_BAD_ARITY,
   BODY_EXPECTED,
@@ skipping 3130 matching lines @@
           howToFix: "Try rewriting the loop as a for-in loop or removing the "
               "'await' keyword.",
           examples: const [
             """
 main() async* {
   await for (int i = 0; i < 10; i++) {}
 }
 """
           ]),
 
+      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.",
           options: const ['--enable-async'],
           howToFix: "Try removing the '#{modifier}' modifier or adding a "
               "body to the method.",
           examples: const [
             """
 abstract class A {
   method() async;
@@ skipping 460 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }