Don't let HCheck instructions prevent GVN.

tests/compiler/dart2js/gvn_test.dart

 // Copyright (c) 2011, 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 'compiler_helper.dart';
 
 const String TEST_ONE = r"""
 void foo(bar) {
   for (int i = 0; i < 1; i++) {
     print(1 + bar);
@@ skipping 23 matching lines @@
 
 // Check that instructions that don't have a builtin equivalent can
 // still be GVN'ed.
 const String TEST_FOUR = r"""
 void foo(a) {
   print(1 >> a);
   print(1 >> a);
 }
 """;
 
+// Check that [HCheck] instructions do not prevent GVN.
+const String TEST_FIVE = r"""
+class A {
+  var foo = 21;
+}
+
+class B {}
+
+main() {
+  var a = [new B(), new A()][0];
+  var b = a.foo;
+  var c = a.foo;
+  if (a is B) {
+    c = a.foo;
+  }
+  return b + c;
+}
+""";
+
 main() {
   String generated = compile(TEST_ONE, entry: 'foo');
   RegExp regexp = new RegExp(r"1 \+ [a-z]+");
   checkNumberOfMatches(regexp.allMatches(generated).iterator, 1);
 
   generated = compile(TEST_TWO, entry: 'foo');
   checkNumberOfMatches(new RegExp("length").allMatches(generated).iterator, 1);
 
   generated = compile(TEST_THREE, entry: 'foo');
   checkNumberOfMatches(new RegExp("number").allMatches(generated).iterator, 1);
 
   generated = compile(TEST_FOUR, entry: 'foo');
   checkNumberOfMatches(new RegExp("shr").allMatches(generated).iterator, 1);
+
+  generated = compileAll(TEST_FIVE);
+  checkNumberOfMatches(
+      new RegExp("get\\\$foo").allMatches(generated).iterator, 1);
 }