Fix receiver refining optimization in the presence of null: any Object method can be called on the …

tests/language/refine_receiver_null_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.
+
+// Regression test for dart2js that used to infer that code following
+// a dynamic call could assume the receiver is not null. This does not
+// work for Object methods.
+
+import "package:expect/expect.dart";
+import "compiler_annotations.dart";
+
+main() {
+  var a = true ? null : 42;
+  a.toString();
+  foo(a);
+}
+
+@DontInline()
+foo(a) {
+  var f = () => 42;
+  Expect.throws(() => a + 42, (e) => e is NoSuchMethodError);
+}