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

tests/language/private_clash_lib.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.
 
-// Script which exits with code 42.
+library a$_b;
 
-import 'dart:io';
+class B {
+  var _c$ = 10; // With library prefix: _a$_b$_c$
 
-void main() {
-  exitCode = 42;
+  getValueA() {
+    try {} catch(e) {} // no inline
+    return this._c$;
+  }
 }