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

tests/language/is_operator_clash_test.dart

-// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
 import "package:expect/expect.dart";
 
 class A {
 }
 
-class B extends A {
+class $B extends A {
 }
 
-class C implements B {
+class C implements $B {
+  // Try to clash with dart2js's isCLASS field.
+  var isB = false;
+  var $isB = false;
+  var is$B = false;
+  var is$$B = false;
+  var $is$B = false;
+
+  var isA = false;
+  var $isA = false;
+  var is$A = false;
+  var is$$A = false;
+  var $is$A = false;
 }
 
 int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
 
 main() {
-  var things = [new A(), new B(), new C()];
+  var things = [new A(), new $B(), new C()];
 
   var a = things[inscrutable(0)];
   Expect.isTrue(a is A);
-  Expect.isFalse(a is B);
+  Expect.isFalse(a is $B);
   Expect.isFalse(a is C);
 
   var b = things[inscrutable(1)];
   Expect.isTrue(b is A);
-  Expect.isTrue(b is B);
+  Expect.isTrue(b is $B);
   Expect.isFalse(b is C);
 
   var c = things[inscrutable(2)];
   Expect.isTrue(c is A);
-  Expect.isTrue(c is B);
+  Expect.isTrue(c is $B);
   Expect.isTrue(c is C);
 }