dart2js: Refactoring, documentation, and a few bugfixes in Namer class.

tests/html/custom_element_name_clash_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.
+
+library custom_elements_name_clash;
+
+import 'dart:html';
+
+class CustomElement extends HtmlElement {
+  factory CustomElement() => new Element.tag('x-custom');
+
+  CustomElement.created() : super.created() {
+  }
+
+  // Try to clash with native 'appendChild' method.
+  var appendChild = 123;
+}
+
+main() {
+  document.registerElement('x-custom', CustomElement);
+  CustomElement custom = new CustomElement();
+  document.body.children.add(custom);
+  // Will call appendChild in JS.
+  custom.children.add(new DivElement()..text = 'Hello world!'); 
+}