Fix returning from async functions.

tests/language/async_control_structures_test.dart

+// Copyright (c) 2014, 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.
+
+// VMOptions=--enable_async
+
+import 'package:expect/expect.dart';
+
+import 'dart:async';
+
+expectThenValue(future, value) {
+  Expect.isTrue(future is Future);
+  future.then((result) {
+    Expect.equals(value, result);
+  });
+}
+
+asyncIf(condition) async {
+  if(condition) {
+    return 1;
+  } else {
+    return 2;
+  }
+  // This return is never reached as the finally block returns from the
+  // function.
+  return 3;
+}
+
+asyncFor(condition) async {
+  for (int i = 0; i < 10; i++) {
+    if (i == 5 && condition) {
+      return 1;
+    }
+  }
+  return 2;
+}
+
+asyncTryCatchFinally(overrideInFinally, doThrow) async {
+  try {
+    if (doThrow) throw 444;
+    return 1;
+  } catch (e) {
+    return e;
+  } finally {
+    if (overrideInFinally) return 3;
+  }
+}
+
+asyncTryCatchLoop() async {
+  var i = 0;
+  var throws = 13;
+  while (true) {
+    try {
+      throw throws;
+    } catch (e) {
+      if (i == throws) { return e; }
+    } finally {
+      i++;
+    }
+  }
+}
+
+asyncImplicitReturn() async {
+  try {}
+  catch (e) {}
+  finally {}
+}
+
+main() {
+  var asyncReturn;
+
+  asyncReturn = asyncIf(true);
+  expectThenValue(asyncReturn, 1);
+  asyncReturn = asyncIf(false);
+  expectThenValue(asyncReturn, 2);
+
+  asyncReturn = asyncFor(true);
+  expectThenValue(asyncReturn, 1);
+  asyncReturn = asyncFor(false);
+  expectThenValue(asyncReturn, 2);
+
+  asyncReturn = asyncTryCatchFinally(true, false);
+  expectThenValue(asyncReturn, 3);
+  asyncReturn = asyncTryCatchFinally(false, false);
+  expectThenValue(asyncReturn, 1);
+  asyncReturn = asyncTryCatchFinally(true, true);
+  expectThenValue(asyncReturn, 3);
+  asyncReturn = asyncTryCatchFinally(false, true);
+  expectThenValue(asyncReturn, 444);
+  asyncReturn = asyncTryCatchLoop();
+  expectThenValue(asyncReturn, 13);
+
+  asyncReturn = asyncImplicitReturn();
+  expectThenValue(asyncReturn, null);
+}