Use better names for captured variables in closures.

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 70d597ef6bd21b18c982608b5eb1fc62019035f3..0a89b42c528461f7a019af081a0d18eec51064c4 100644
--- a/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
+++ b/pkg/compiler/lib/src/js_backend/field_naming_mixin.dart
@@ -22,9 +22,7 @@ abstract class _MinifiedFieldNamer implements Namer {
     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);
     }
@@ -77,7 +75,7 @@ class _FieldNamingRegistry {
         nameStore.add(
             new StringBackedName(MinifyNamer._reservedNativeProperties[index]));
       } else {
-        nameStore.add(namer.getFreshName(NamingScope.instance, "field$index"));
+        nameStore.add(namer.getFreshName(namer.instanceScope, "field$index"));
       }
     }