Couple of backend fixes:

tests/compiler/dart2js_extra/mirror_type_inference_field2_test.dart

Index: tests/compiler/dart2js_extra/mirror_type_inference_field2_test.dart
===================================================================
--- tests/compiler/dart2js_extra/mirror_type_inference_field2_test.dart	(revision 0)
+++ tests/compiler/dart2js_extra/mirror_type_inference_field2_test.dart	(revision 0)
@@ -0,0 +1,23 @@
+// 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.
+
+// Test that type inference sees the possible modification of `fisk` from
+// the mirror system and not infers the value to be an integer, or any
+// other elements that use `fisk` like `otherFisk` to be of type integer.
+library test;
+
+@MirrorsUsed(targets: 'fisk', override: '*')
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+var fisk = 1;
+var otherFisk = 42;
+
+main() {
+  var lm = currentMirrorSystem().findLibrary(const Symbol('test')).single;
+  lm.setField(const Symbol('fisk'), 'hest');
+  otherFisk = fisk;
+  Expect.isTrue(otherFisk is String);
+}