Use better names for captured variables in closures.

pkg/compiler/lib/src/js_backend/field_naming_mixin.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.
 
 part of js_backend;
 
 abstract class _MinifiedFieldNamer implements Namer {
   _FieldNamingRegistry get fieldRegistry;
 
   // Returns a minimal name for the field that is globally unique along
   // the given element's class inheritance chain.
   //
   // The inheritance scope based naming might not yield a name. For instance,
   // this could be because the field belongs to a mixin. In such a case this
   // will return `null` and a normal field name has to be used.
   jsAst.Name _minifiedInstanceFieldPropertyName(Element element) {
     if (backend.hasFixedBackendName(element)) {
       return new StringBackedName(backend.getFixedBackendName(element));
     }
 
     _FieldNamingScope names;
     if (element is BoxFieldElement) {
       names = new _FieldNamingScope.forBox(element.box, fieldRegistry);
     } else {
-      ClassElement cls = element is ClosureFieldElement
-          ? element.closureClass
-          : element.enclosingClass;
+      ClassElement cls = element.enclosingClass;
       names =
           new _FieldNamingScope.forClass(cls, compiler.world, fieldRegistry);
     }
 
     if (names.containsField(element)) {
       return names[element];
     }
     return null;
   }
 }
@@ skipping 32 matching lines @@
       // The namer usually does not use certain names as they clash with
       // existing properties on JS objects (see [_reservedNativeProperties]).
       // However, some of them are really short and safe to use for fields.
       // Thus, we shortcut the namer to use those first.
       assert(index == nameStore.length);
       if (index < MinifyNamer._reservedNativeProperties.length &&
           MinifyNamer._reservedNativeProperties[index].length <= 2) {
         nameStore.add(
             new StringBackedName(MinifyNamer._reservedNativeProperties[index]));
       } else {
-        nameStore.add(namer.getFreshName(NamingScope.instance, "field$index"));
+        nameStore.add(namer.getFreshName(namer.instanceScope, "field$index"));
       }
     }
 
     return nameStore[index];
   }
 }
 
 /**
  * A [_FieldNamingScope] encodes a node in the inheritance tree of the current
  * class hierarchy. The root node typically is the node corresponding to the
@@ skipping 137 matching lines @@
       : super.rootScope(box, registry);
 
   @override
   bool containsField(_) => true;
 
   jsAst.Name operator [](Element field) {
     if (!names.containsKey(field)) add(field);
     return names[field];
   }
 }