Adds check for `AsExpression`s (Fix #145).

test/rules/avoid_as.dart

 // Copyright (c) 2015, 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 'dart:math' as dartMath; //LINT [23:8]
+main() {
+  var pm;
+  try {
+    (pm as Person).firstName = 'Seth'; //LINT
+  } on CastError { }
 
-main() {
-  print(dartMath.PI);
+  Person person = pm;
+  person.firstName = 'Seth';
 }
+
+class Person {
+  var firstName;
+}