Copy the parameters to a sync* function for each invocation of .iterator

pkg/compiler/lib/src/js/rewrite_async.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 rewrite_async;
 
 import "dart:math" show max;
 import 'dart:collection';
 
 import 'package:_internal/compiler/js_lib/shared/async_await_error_codes.dart'
@@ skipping 1752 matching lines @@
   void addYield(js.DartYield node, js.Expression expression) {
     if (node.hasStar) {
       addStatement(
           new js.Return(new js.Call(yieldStarExpression, [expression])));
     } else {
       addStatement(new js.Return(expression));
     }
   }
 
   @override
-  js.Fun finishFunction(List<js.Parameter> params,
+  js.Fun finishFunction(List<js.Parameter> parameters,
                         js.Statement rewrittenBody,
                         js.VariableDeclarationList variableDeclarations) {
+    // Each iterator invocation on the iterable should work on its own copy of
+    // the parameters.
+    List<js.VariableInitialization> declarations =
+        new List<js.VariableInitialization>();
+    List<js.Parameter> renamedParameters = new List<js.Parameter>();
+    for (js.Parameter parameter in parameters) {
+      String name = parameter.name;
+      String renamedName = freshName(name);
+      renamedParameters.add(new js.Parameter(renamedName));
+      declarations.add(
+          new js.VariableInitialization(new js.VariableDeclaration(name),
+                                        new js.VariableUse(renamedName)));
+    }
+    js.VariableDeclarationList copyParameters =

[floitsch, 2015/03/04 13:03:02]

Add a TODO that you don't need to do this if the parameters aren't mutated.

[sigurdm, 2015/03/04 13:05:47]

Done.

+        new js.VariableDeclarationList(declarations);
     return js.js("""
-          function (#params) {
+          function (#renamedParameters) {
             if (#needsThis)
               var #self = this;
             return new #newIterable(function () {
+              if (#hasParameters) {
+                #copyParameters;
+              }
               #varDecl;
               return function #body(#errorCode, #result) {
                 if (#errorCode === #ERROR) {
                     #currentError = #result;
                     #goto = #handler;
                 }
                 #helperBody;
               };
             });
           }
           """, {
-            "params": params,
+            "renamedParameters": renamedParameters,
             "needsThis": analysis.hasThis,
             "helperBody": rewrittenBody,
+            "hasParameters": parameters.isNotEmpty,
+            "copyParameters": copyParameters,
             "varDecl": variableDeclarations,
             "errorCode": errorCodeName,
             "newIterable": newIterable,
             "body": bodyName,
             "self": selfName,
             "result": resultName,
             "goto": goto,
             "handler": handler,
             "currentError": currentErrorName,
             "ERROR": js.number(error_codes.ERROR),
@@ skipping 648 matching lines @@
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     hasYield = true;
     visit(node.expression);
     return true;
   }
 }