Bailout on array optimizations if the array might escape to reflection land.

tests/lib/mirrors/array_tracing2_test.dart

+// Copyright (c) 2013, 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:mirrors';
+
+class A {
+  var field;
+}
+
+main() {
+  var a = new A();
+  var mirror = reflect(a);
+  var array = [42];
+  a.field = array;
+  var field = mirror.getField(#field);
+  field.invoke(#clear, []);
+  if (array.length == 1) throw 'Test failed';
+}