Fix loop-invariant code motion.

tests/language/loop_hoist_test.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.
+
+class A {
+  num x;
+  A() : this.x = 0;
+
+  void bar(){
+    // dart2js hoisted the this.x out of the loop, and missed that setX would
+    // change the value.
+    for (int i = 1; i < 3; i++) {
+      setX(499);
+      foo(x);
+      break;
+    }
+  }
+  setX(x) => this.x = x;
+}
+
+var saved;
+foo(x) => saved = x;
+
+main() {
+  A a = new A();
+  for (int i = 0; i < 1; i++) {
+    a.bar();
+  }
+  Expect.equals(499, saved);
+}