Allow `return;` in async method.

tests/compiler/dart2js/type_checker_test.dart

Index: tests/compiler/dart2js/type_checker_test.dart
diff --git a/tests/compiler/dart2js/type_checker_test.dart b/tests/compiler/dart2js/type_checker_test.dart
index 1d717e53ce2520e831afdeabb340b267e8337395..939640d559660c0bfacbdec70eb749d274ece8a8 100644
--- a/tests/compiler/dart2js/type_checker_test.dart
+++ b/tests/compiler/dart2js/type_checker_test.dart
@@ -2038,7 +2038,7 @@ testAsyncReturn(MockCompiler compiler) {
     return analyzeTopLevel(code, expectedWarnings);
   }
   return Future.wait([
-    check("Future<int> foo() async { return; }", MessageKind.RETURN_NOTHING),
+    check("Future<int> foo() async { return; }"),
     check("Future<int> foo() async { return null; }"),
     check("Future<int> foo() async { return 0; }"),
     check("Future<int> foo() async { return ''; }", NOT_ASSIGNABLE),
@@ -2053,7 +2053,7 @@ testAsyncReturn(MockCompiler compiler) {
     check("void foo() async { return 0; }", MessageKind.RETURN_VALUE_IN_VOID),
     check("void foo() async { return new Future.value(); }",
           MessageKind.RETURN_VALUE_IN_VOID),
-    check("int foo() async { return; }", MessageKind.RETURN_NOTHING),
+    check("int foo() async { return; }"),
     check("int foo() async { return 0; }", NOT_ASSIGNABLE),
     check("int foo() async { return new Future<int>.value(); }",
           NOT_ASSIGNABLE),