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

tests/compiler/dart2js_extra/checked_accessor_test.dart

Index: tests/compiler/dart2js_extra/checked_accessor_test.dart
diff --git a/tests/compiler/dart2js_extra/checked_accessor_test.dart b/tests/compiler/dart2js_extra/checked_accessor_test.dart
index cda70a7260c17f4bb9f4f01726313b3345942e20..f20091fcd1e59376f10830408a6b38c501c13134 100644
--- a/tests/compiler/dart2js_extra/checked_accessor_test.dart
+++ b/tests/compiler/dart2js_extra/checked_accessor_test.dart
@@ -7,7 +7,6 @@ class E {
 }
 
 class WithGetter {
-  //   get field => null;
   String field;
 }
 
@@ -15,8 +14,9 @@ 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';
+  });
 }