Use better names for captured variables in closures.

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

Index: pkg/compiler/lib/src/js_backend/frequency_namer.dart
diff --git a/pkg/compiler/lib/src/js_backend/frequency_namer.dart b/pkg/compiler/lib/src/js_backend/frequency_namer.dart
index 0aaf8f6703737c9e5bf4d06f6f67cb8124f96b31..6fa2ccf8ec9e6b513baf7007a15cfdd016f94341 100644
--- a/pkg/compiler/lib/src/js_backend/frequency_namer.dart
+++ b/pkg/compiler/lib/src/js_backend/frequency_namer.dart
@@ -26,7 +26,7 @@ class FrequencyBasedNamer extends Namer with _MinifiedFieldNamer,
   }
 
   TokenScope newScopeFor(NamingScope scope) {
-    if (scope == NamingScope.instance) {
+    if (scope == instanceScope) {
       Set<String> illegalNames = new Set<String>.from(jsReserved);
       for (String illegal in MinifyNamer._reservedNativeProperties) {
         illegalNames.add(illegal);
@@ -50,8 +50,7 @@ class FrequencyBasedNamer extends Namer with _MinifiedFieldNamer,
 
     // Get the name the normal namer would use as a key.
     String proposed = _generateFreshStringForName(proposedName,
-                                                  getUsedNames(scope),
-                                                  getSuggestedNames(scope),
+                                                  scope,
                                                   sanitizeForNatives:
                                                   sanitizeForNatives,
                                                   sanitizeForAnnotations: