Typecheck return with respect to async.

pkg/compiler/lib/src/warnings.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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 2114 matching lines @@
           howToFix: "Try removing the '#{modifier}' modifier.",
           examples: const ["""
 class A {
   A() async;
 }
 main() => new A();""",
 
 """
 class A {
   A();
-  factory A.a() async* => new A();
+  factory A.a() async* {}
 }
 main() => new A.a();"""]);
 
   static const MessageKind ASYNC_MODIFIER_ON_SETTER =
       const MessageKind(
           "The modifier '#{modifier}' is not allowed on setters.",
           options: const ['--enable-async'],
           howToFix: "Try removing the '#{modifier}' modifier.",
           examples: const ["""
 class A {
@@ skipping 23 matching lines @@
 }""",
 """
 main() async* {
  var yield;
 }""",
 """
 main() sync* {
  var yield;
 }"""]);
 
+  static const MessageKind RETURN_IN_GENERATOR =
+      const MessageKind(
+          "'return' with a value is not allowed in a method body using the "
+          "'#{modifier}' modifier.",
+          howToFix: "Try removing the value, replacing 'return' with 'yield' "
+                    "or changing the method body modifier.",
+          examples: const [
+"""
+foo() async* { return 0; }
+main() => foo();
+""",
+
+"""
+foo() sync* { return 0; }
+main() => foo();
+"""]);
+
   static const MessageKind NATIVE_NOT_SUPPORTED = const MessageKind(
       "'native' modifier is not supported.",
       howToFix: "Try removing the 'native' implementation or analyzing the "
                 "code with the --allow-native-extensions option.",
       examples: const ["""
 main() native "Main";
 """]);
 
   static const MessageKind DART_EXT_NOT_SUPPORTED = const MessageKind(
       "The 'dart-ext' scheme is not supported.",
@@ skipping 253 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';
   }
 }