dart2js: Implement frequency based naming.

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

Index: pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
diff --git a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
index 1666ca52da706d712442797575ec0a188586ee41..707e7e7636929fe89ff84514e6d5a3e390f5bd20 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -44,7 +44,7 @@ abstract class _MinifiedFieldNamer implements Namer {
  * hierarchy is encoded using instances of [_FieldNamingScope].
  */
 class _FieldNamingRegistry {
-  final MinifyNamer namer;
+  final Namer namer;
 
   final Map<Entity, _FieldNamingScope> scopes =
       new Map<Entity, _FieldNamingScope>();
@@ -77,8 +77,7 @@ class _FieldNamingRegistry {
         nameStore.add(
             new StringBackedName(MinifyNamer._reservedNativeProperties[index]));
       } else {
-        nameStore.add(namer.getFreshName("field$index", namer.usedInstanceNames,
-            namer.suggestedInstanceNames));
+        nameStore.add(namer.getFreshName(NamingScope.instance, "field$index"));
       }
     }