Refactor BackendHelpers to make 'find' methods private.

pkg/compiler/lib/src/js_backend/backend_impact.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.
 
 library dart2js.js_helpers.impact;
 
 import '../common/names.dart';
 import '../compiler.dart' show Compiler;
 import '../core_types.dart' show CommonElements;
 import '../dart_types.dart' show InterfaceType;
@@ skipping 251 matching lines @@
     ], otherImpacts: [
       _needsInt('Needed to encode the invocation kind of super.noSuchMethod.'),
       _needsList('Needed to encode the arguments of super.noSuchMethod.'),
       _needsString('Needed to encode the name of super.noSuchMethod.')
     ]);
   }
 
   BackendImpact _constantMapLiteral;
 
   BackendImpact get constantMapLiteral {
-    if (_constantMapLiteral == null) {
-      ClassElement find(String name) {
-        return helpers.find(helpers.jsHelperLibrary, name);
-      }
-
-      _constantMapLiteral = new BackendImpact(instantiatedClasses: [
-        find(JavaScriptMapConstant.DART_CLASS),
-        find(JavaScriptMapConstant.DART_PROTO_CLASS),
-        find(JavaScriptMapConstant.DART_STRING_CLASS),
-        find(JavaScriptMapConstant.DART_GENERAL_CLASS)
-      ]);
-    }
-    return _constantMapLiteral;
+    return _constantMapLiteral ??= new BackendImpact(instantiatedClasses: [
+      helpers.constantMapClass,
+      helpers.constantProtoMapClass,
+      helpers.constantStringMapClass,
+      helpers.generalConstantMapClass,
+    ]);
   }
 
   BackendImpact _symbolConstructor;
 
   BackendImpact get symbolConstructor {
     return _symbolConstructor ??=
         new BackendImpact(staticUses: [helpers.symbolValidatedConstructor]);
   }
 
   BackendImpact _constSymbol;
@@ skipping 435 matching lines @@
 
   BackendImpact _staticClosure;
 
   /// Backend impact for performing closurization of a top-level or static
   /// function.
   BackendImpact get staticClosure {
     return _staticClosure ??=
         new BackendImpact(globalClasses: [helpers.closureClass]);
   }
 }