Fix ddc perf issue

pkg/dev_compiler/lib/src/compiler/js_names.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.
 
 import 'dart:collection';
 
 import '../js_ast/js_ast.dart';
 
 /// The ES6 name for the Dart SDK.  All dart:* libraries are in this module.
 const String dartSdkModule = 'dart_sdk';
@@ skipping 160 matching lines @@
   void _finishFunctions() {
     scope.functions.forEach((FunctionExpression f, _FunctionScope s) {
       scope = s;
       super.visitFunctionExpression(f);
       _finishFunctions();
       scope = scope.parent;
     });
   }
 
   void _finishNames() {
-    var allNames = new Set<String>();
     pendingRenames.forEach((id, scopes) {
-      allNames.clear();
-      for (var s in scopes) allNames.addAll(s.used);
-
-      var name = _findName(id, allNames);
+      var name = _findName(id, scopes);
       for (var s in scopes) {
         s.used.add(name);
         s.renames[id] = name;
       }
     });
   }
 
-  static String _findName(Object id, Set<String> usedNames) {
+  static String _findName(Object id, Set<_FunctionScope> scopes) {
     String name;
     bool valid;
     if (id is TemporaryId) {
       name = id.name;
       valid = !invalidVariableName(name);
     } else {
       name = id;
       valid = false;
     }
 
     // Try to use the temp's name, otherwise rename.
     String candidate;
-    if (valid && !usedNames.contains(name)) {
+    if (valid && !scopes.any((scope) => scope.used.contains(name))) {
       candidate = name;
     } else {
       // This assumes that collisions are rare, hence linear search.
       // If collisions become common we need a better search.
       // TODO(jmesserly): what's the most readable scheme here? Maybe 1-letter
       // names in some cases?
       candidate = name == 'function' ? 'func' : '${name}\$';
-      for (int i = 0; usedNames.contains(candidate); i++) {
+      for (int i = 0;
+          scopes.any((scope) => scope.used.contains(candidate));
+          i++) {
         candidate = '${name}\$$i';
       }
     }
     return candidate;
   }
 }
 
 bool needsRename(Identifier node) =>
     node is TemporaryId || node.allowRename && invalidVariableName(node.name);
 
@@ skipping 65 matching lines @@
 bool invalidStaticFieldName(String name) {
   switch (name) {
     case "arguments":
     case "caller":
     case "callee":
     case "name":
       return true;
   }
   return false;
 }