Refactor BackendHelpers to make 'find' methods private.

pkg/compiler/lib/src/native/enqueue.dart

 // Copyright (c) 2014, 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.
 
 import 'dart:collection' show Queue;
 
 import '../common.dart';
 import '../common/backend_api.dart' show ForeignResolver;
 import '../common/resolution.dart' show Resolution;
 import '../compiler.dart' show Compiler;
@@ skipping 265 matching lines @@
     return _annotationReturnsClass;
   }
 
   ClassElement get annotationJsNameClass {
     findAnnotationClasses();
     return _annotationJsNameClass;
   }
 
   void findAnnotationClasses() {
     if (_annotationCreatesClass != null) return;
-    ClassElement find(name) {
-      Element e = helpers.findHelper(name);
-      if (e == null || e is! ClassElement) {
-        reporter.internalError(NO_LOCATION_SPANNABLE,
-            "Could not find implementation class '${name}'.");
-      }
-      return e;
-    }
 
-    _annotationCreatesClass = find('Creates');
-    _annotationReturnsClass = find('Returns');
-    _annotationJsNameClass = find('JSName');
+    _annotationCreatesClass = helpers.annotationCreatesClass;
+    _annotationReturnsClass = helpers.annotationReturnsClass;
+    _annotationJsNameClass = helpers.annotationJSNameClass;
   }
 
   /// Returns the JSName annotation string or `null` if no JSName annotation is
   /// present.
   String findJsNameFromAnnotation(Element element) {
     String name = null;
     ClassElement annotationClass = annotationJsNameClass;
     for (MetadataAnnotation annotation in element.implementation.metadata) {
       annotation.ensureResolved(resolution);
       ConstantValue value =
@@ skipping 183 matching lines @@
       reporter.log('All native types marked as used due to $cause.');
     }
   }
 
   Iterable<ClassElement> _findUnusedClassesMatching(
       bool predicate(classElement)) {
     return _unusedClasses.where(predicate);
   }
 
   Iterable<ClassElement> _onFirstNativeClass(WorldImpactBuilder impactBuilder) {
-    void staticUse(name) {
-      Element element = helpers.findHelper(name);
+    void staticUse(element) {
       impactBuilder.registerStaticUse(new StaticUse.foreignUse(element));
       backend.registerBackendUse(element);
       compiler.globalDependencies.registerDependency(element);
     }
 
-    staticUse('defineProperty');
-    staticUse('toStringForNativeObject');
-    staticUse('hashCodeForNativeObject');
-    staticUse('convertDartClosureToJS');
+    staticUse(helpers.defineProperty);
+    staticUse(helpers.toStringForNativeObject);
+    staticUse(helpers.hashCodeForNativeObject);
+    staticUse(helpers.closureConverter);
     return _findNativeExceptions();
   }
 
   Iterable<ClassElement> _findNativeExceptions() {
     return _findUnusedClassesMatching((classElement) {
       // TODO(sra): Annotate exception classes in dart:html.
       String name = classElement.name;
       if (name.contains('Exception')) return true;
       if (name.contains('Error')) return true;
       return false;
@@ skipping 140 matching lines @@
     List<Element> directSubtypes =
         emitter.directSubtypes.putIfAbsent(superclass, () => <ClassElement>[]);
     directSubtypes.add(cls);
   }
 
   void logSummary(log(message)) {
     log('Compiled ${_registeredClasses.length} native classes, '
         '${_unusedClasses.length} native classes omitted.');
   }
 }