Fix renaming of exception variables on left-hand-sides.

tests/language/async_await_catch_regression_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.
 
-// Test that an async function does not start immediately.
-
 import "package:expect/expect.dart";
 import "package:async_helper/async_helper.dart";
 
-var x = 0;
-
 foo() async {
-  x++;
-  await 1;
-  x++;
+  throw 42;
 }
 
-void main() {
+test() async {
+  var exception;
+  try {
+    await foo();
+  } catch (e) {
+    print(await(e));
+    await (exception = await e);
+  }
+  Expect.equals(42, exception);
+}
+
+main() {
   asyncStart();
-  foo().then((_) => Expect.equals(2, x)).whenComplete(asyncEnd);
-  Expect.equals(0, x);
-}
+  test().then((_) => asyncEnd());
+}