dart2js: Use an abstract Name class for names in the generated JavaScript ast.

pkg/compiler/lib/src/js_backend/js_backend.dart

Index: pkg/compiler/lib/src/js_backend/js_backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/js_backend.dart b/pkg/compiler/lib/src/js_backend/js_backend.dart
index 7ca701a2b8416e6896afd6ad83e0d23e12244c0b..2664635ca5771d0ca5f22f974a5805c15b14088c 100644
--- a/pkg/compiler/lib/src/js_backend/js_backend.dart
+++ b/pkg/compiler/lib/src/js_backend/js_backend.dart
@@ -5,6 +5,7 @@
 library js_backend;
 
 import 'dart:async' show EventSink, Future;
+import 'dart:collection' show HashMap;
 
 import 'package:_internal/compiler/js_lib/shared/embedded_names.dart'
     as embeddedNames;
@@ -54,6 +55,7 @@ part 'checked_mode_helpers.dart';
 part 'constant_emitter.dart';
 part 'constant_handler_javascript.dart';
 part 'custom_elements_analysis.dart';
+part 'frequency_namer.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'no_such_method_registry.dart';