Avoid syntax errors on oddly named fields.

dart/tests/html/track_element_constructor_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.
+
+// A regression test for dart2js generating illegal JavaScript code
+// dynamically in non-csp mode.  The name of the field "defaultValue"
+// in JavaScript is "default".  This meant that dart2js would create a
+// constructor function that looked like this:
+//
+// function TrackElement(default) { this.default = default; }
+
+import 'dart:html';
+
+void main() {
+  document.body.append(new TrackElement()..defaultValue = true);
+  if (!document.query('track').defaultValue) {
+    throw 'Expected default value to be true';
+  }
+  document.query('track').defaultValue = false;
+  if (document.query('track').defaultValue) {
+    throw 'Expected default value to be false';
+  }
+}