Refine the type of a local after it has been used as the receiver of a call.

tests/language/inference_captured_variable2_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.
+
+// Ensure that dart2js's receiver specialization optimization works
+// with captured variables.
+
+import "package:expect/expect.dart";
+
+var list = [new Object(), 31];
+
+main() {
+  Expect.throws(() => foo()() + 42, (e) => e is NoSuchMethodError);
+}
+
+foo() {
+  var a = list[0];
+  var closure = (() => a - 42);
+  return () => a + 54;
+}