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

tests/compiler/dart2js_extra/checked_accessor_test.dart

 // Copyright (c) 2013, 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.
 
 class E {
   missingType field;
 }
 
 class WithGetter {
-  //   get field => null;
   String field;
 }
 
 void main() {
   f(x) {
     x.field = true;
   }
-
-  [new E(), new WithGetter()].forEach(f);
-  new missingType();
-  new E().field = 'a string';
+  Expect.throws(() {
+    [new E(), new WithGetter()].forEach(f);
+    new missingType();
+    new E().field = 'a string';
+  });
 }