Fix dart2js by copying keys of a map that is modified under us.

sdk/lib/_internal/compiler/implementation/closure.dart

 // Copyright (c) 2012, 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 closureToClassMapper;
 
 import "elements/elements.dart";
 import "dart2jslib.dart";
 import "scanner/scannerlib.dart" show Token;
 import "tree/tree.dart";
@@ skipping 241 matching lines @@
   void updateClosures() {
     for (Expression closure in closures) {
       // The captured variables that need to be stored in a field of the closure
       // class.
       Set<Element> fieldCaptures = new Set<Element>();
       Set<Element> boxes = new Set<Element>();
       ClosureClassMap data = closureMappingCache[closure];
       Map<Element, Element> freeVariableMapping = data.freeVariableMapping;
       // We get a copy of the keys and iterate over it, to avoid modifications
       // to the map while iterating over it.
-      freeVariableMapping.keys.forEach((Element fromElement) {
+      freeVariableMapping.keys.toList().forEach((Element fromElement) {
         assert(fromElement == freeVariableMapping[fromElement]);
         Element updatedElement = capturedVariableMapping[fromElement];
         assert(updatedElement != null);
         if (fromElement == updatedElement) {
           assert(freeVariableMapping[fromElement] == updatedElement);
           assert(Elements.isLocal(updatedElement)
                  || updatedElement.isTypeVariable());
           // The variable has not been boxed.
           fieldCaptures.add(updatedElement);
         } else {
@@ skipping 378 matching lines @@
   }
 
   visitTryStatement(TryStatement node) {
     // TODO(ngeoffray): implement finer grain state.
     bool oldInTryStatement = inTryStatement;
     inTryStatement = true;
     node.visitChildren(this);
     inTryStatement = oldInTryStatement;
   }
 }