In LoadOptimizer::EmitPhis check for environment uses of phis.

tests/language/store_to_load_forwarding_phis_vm_test.dart

+// 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 correct handling of phis with only environment uses that were inserted
+// by store to load forwarding.
+// VMOptions=--optimization_counter_threshold=100
+
+import 'dart:async';
+
+class A {
+  var _foo;
+
+  get foo {
+    if (_foo == null) {
+      _foo = new A();
+    }
+    return _foo;
+  }
+}
+
+foo(obj) {
+  var a = obj.foo;
+  return new Future.immediate(null).then((val) { });
+}
+
+main() {
+  final obj = new A();
+  for (var i = 0; i < 200; i++) {
+    foo(obj);
+  }
+}