Use better names for captured variables in closures.

pkg/compiler/lib/src/js_backend/frequency_namer.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.
 
 part of js_backend;
 
 class FrequencyBasedNamer extends Namer with _MinifiedFieldNamer,
     _MinifiedOneShotInterceptorNamer implements jsAst.TokenFinalizer {
   _FieldNamingRegistry fieldRegistry;
   List<TokenName> tokens = new List<TokenName>();
 
   Map<NamingScope, TokenScope> _tokenScopes =
       new Maplet<NamingScope, TokenScope>();
 
   // Some basic settings for smaller names
   String get isolateName => 'I';
   String get isolatePropertiesName => 'p';
   bool get shouldMinify => true;
 
   final String getterPrefix = 'g';
   final String setterPrefix = 's';
   final String callPrefix = ''; // this will create function names $<n>
 
   FrequencyBasedNamer(Compiler compiler) : super(compiler) {
     fieldRegistry = new _FieldNamingRegistry(this);
   }
 
   TokenScope newScopeFor(NamingScope scope) {
-    if (scope == NamingScope.instance) {
+    if (scope == instanceScope) {
       Set<String> illegalNames = new Set<String>.from(jsReserved);
       for (String illegal in MinifyNamer._reservedNativeProperties) {
         illegalNames.add(illegal);
         if (MinifyNamer._hasBannedPrefix(illegal)) {
           illegalNames.add(illegal.substring(1));
         }
       }
       return new TokenScope(illegalNames);
     } else {
       return new TokenScope(jsReserved);
     }
   }
 
   @override
   jsAst.Name getFreshName(NamingScope scope, String proposedName,
                           {bool sanitizeForNatives: false,
                            bool sanitizeForAnnotations: false}) {
     // Grab the scope for this token
     TokenScope tokenScope = _tokenScopes.putIfAbsent(scope,
                                                      () => newScopeFor(scope));
 
     // Get the name the normal namer would use as a key.
     String proposed = _generateFreshStringForName(proposedName,
-                                                  getUsedNames(scope),
-                                                  getSuggestedNames(scope),
+                                                  scope,
                                                   sanitizeForNatives:
                                                   sanitizeForNatives,
                                                   sanitizeForAnnotations:
                                                   sanitizeForAnnotations);
 
     TokenName name = new TokenName(tokenScope, proposed);
     tokens.add(name);
     return name;
   }
 
@@ skipping 68 matching lines @@
     do {
       proposal = new String.fromCharCodes(_nextName);
       _incrementName();
     } while (MinifyNamer._hasBannedPrefix(proposal) ||
              illegalNames.contains(proposal));
 
     return proposal;
   }
 }