Refactor BackendHelpers to make 'find' methods private.

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

 // Copyright (c) 2012, 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.
 
 library dart2js.constant_system.js;
 
 import '../compiler.dart' show Compiler;
 import '../constant_system_dart.dart';
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 352 matching lines @@
     }
 
     bool hasProtoKey = (protoValue != null);
     DartType keysType;
     if (sourceType.treatAsRaw) {
       keysType = commonElements.listType();
     } else {
       keysType = commonElements.listType(sourceType.typeArguments.first);
     }
     ListConstantValue keysList = new ListConstantValue(keysType, keys);
-    String className = onlyStringKeys
+    ClassElement classElement = onlyStringKeys
         ? (hasProtoKey
-            ? JavaScriptMapConstant.DART_PROTO_CLASS
-            : JavaScriptMapConstant.DART_STRING_CLASS)
-        : JavaScriptMapConstant.DART_GENERAL_CLASS;
-    ClassElement classElement = backend.helpers.jsHelperLibrary.find(className);
+            ? backend.helpers.constantProtoMapClass
+            : backend.helpers.constantStringMapClass)
+        : backend.helpers.generalConstantMapClass;
     classElement.ensureResolved(compiler.resolution);
     List<DartType> typeArgument = sourceType.typeArguments;
     InterfaceType type;
     if (sourceType.treatAsRaw) {
       type = classElement.rawType;
     } else {
       type = new InterfaceType(classElement, typeArgument);
     }
     return new JavaScriptMapConstant(
         type, keysList, values, protoValue, onlyStringKeys);
@@ skipping 51 matching lines @@
       result.add(keyList);
     } else {
       // Add the keys individually to avoid generating an unused list constant
       // for the keys.
       result.addAll(keys);
     }
     result.addAll(values);
     return result;
   }
 }