Allow `return <expr>` in sync* and async* javascript.

tests/language/syncstar_less_than_test.dart

+// Copyright (c) 2015, the Dart Team. All rights reserved. Use of this
+// source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+import "package:expect/expect.dart";
+
+confuse(x) => [1, 'x', true, null, x].last;

[floitsch, 2015/07/15 14:55:00]

The expect package has annotations to assume 'dynamic' for return types.
'.last' on a list-literal is way too easy to optimize.

+
+Iterable<int> foo() sync* {
+ var a = confuse(1);
+ if (a < 10) {
+   yield 2;
+ }
+}
+
+main() {
+  Expect.listEquals(foo().toList(), [2]);
+}