dart2js cps: Replace getter/setter calls with direct field access.

tests/language/dead_field_access_test.dart

 // Copyright (c) 2015, 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.
 
-import 'dart:math' as Math;
 import 'package:expect/expect.dart';
 
-main() {
-  Expect.equals(4, new C().m());
+class Foo {
+  var field = 10;
 }
 
-class C {
-  max(a) => a;
+@NoInline()
+getField(x) {
+  x.field;
+  return 34;
+}
 
-  m() {
-    return max(Math.max(2, 4));
-  }
-}
+main() {
+  Expect.equals(34, getField(new Foo()));
+  Expect.throws(() => getField(null));
+}