fixes #221, initial sync*, async, async* implementation

test/codegen/language/async_star_cancel_and_throw_in_finally_test.dart

+// Copyright (c) 2015, 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.
+
+import"dart:async";
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+// When an exception is thrown in the finally block cleaning up after a cancel,
+// the future returned from cancel should complete with an error.
+
+foo() async* {
+  try {
+    int i = 0;
+    while(true) {
+      yield i++;
+    }
+  } finally {
+    throw "Error";
+  }
+}
+
+test() async {
+  var completer = new Completer();
+  var s;
+  s = foo().listen((e) async {
+    Expect.equals(0, e);
+    try {
+      await s.cancel();
+      Expect.fail("Did not throw");
+    } catch(e) {
+      Expect.equals("Error", e);
+      completer.complete();
+    }
+  });
+  await completer.future;
+}
+
+main() {
+  asyncStart();
+  test().then((_) => asyncEnd());
+}