Split js_backend into multiple libraries

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;
+part of js_backend.namer;
 
 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>();
 
@@ skipping 115 matching lines @@
     String proposal;
     do {
       proposal = new String.fromCharCodes(_nextName);
       _incrementName();
     } while (MinifyNamer._hasBannedPrefix(proposal) ||
         illegalNames.contains(proposal));
 
     return proposal;
   }
 }