fix temps that have the same name to have different Elements

lib/src/codegen/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.
 
 library dev_compiler.src.codegen.js_names;
 
 import 'dart:collection';
 import 'package:dev_compiler/src/js/js_ast.dart';
 
 /// Unique instance for temporary variables. Will be renamed consistently
@@ skipping 67 matching lines @@
 
   _RenameVisitor.build(Node root) {
     scope = rootScope;
     root.accept(this);
     _finishFunctions();
     _finishNames();
   }
 
   declare(Identifier node) {
     var id = identifierKey(node);
-    scope.declared.add(id);
+    var notAlreadyDeclared = scope.declared.add(id);
+    // Normal identifiers can be declared multiple times, because we don't
+    // implement block scope yet. However temps should only be declared once.
+    assert(notAlreadyDeclared || node is! TemporaryId);
     _markUsed(node, id, scope);
   }
 
   visitIdentifier(Identifier node) {
     var id = identifierKey(node);
 
     // Find where the node was declared.
     var declScope = scope;
     while (declScope != null && !declScope.declared.contains(id)) {
       declScope = declScope.parent;
@@ skipping 136 matching lines @@
 /// In particular, "caller" "callee" and "arguments" cannot be used.
 bool invalidStaticFieldName(String name) {
   switch (name) {
     case "arguments":
     case "caller":
     case "callee":
       return true;
   }
   return false;
 }