Refactor BackendHelpers to make 'find' methods private.

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

Index: pkg/compiler/lib/src/js_backend/backend_impact.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend_impact.dart b/pkg/compiler/lib/src/js_backend/backend_impact.dart
index 2a3244c03407f2f1246a19ae3fb322809b5e1737..92c6fc95021d990ddd53e94cd14171eaea37c783 100644
--- a/pkg/compiler/lib/src/js_backend/backend_impact.dart
+++ b/pkg/compiler/lib/src/js_backend/backend_impact.dart
@@ -269,19 +269,12 @@ class BackendImpacts {
   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;