Fix regression on constant folding an interceptor into a dynamic invocation.

tests/language/getter_setter_interceptor_test.dart

Index: tests/language/getter_setter_interceptor_test.dart
===================================================================
--- tests/language/getter_setter_interceptor_test.dart	(revision 0)
+++ tests/language/getter_setter_interceptor_test.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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 {
+  int length = 0;
+
+  foo() => length++;
+  bar() => ++length;
+}
+
+main() {
+  Expect.equals(0, new A().foo());
+  Expect.equals(1, new A().bar());
+}