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

test/rules/avoid_as.dart

Index: test/rules/avoid_as.dart
diff --git a/test/rules/library_prefixes.dart b/test/rules/avoid_as.dart
similarity index 57%
copy from test/rules/library_prefixes.dart
copy to test/rules/avoid_as.dart
index 548b875ef3b47c951814e8e5bf7a8faffaf14907..d79b052e01dde05243a471a6478d3b70b7244523 100644
--- a/test/rules/library_prefixes.dart
+++ b/test/rules/avoid_as.dart
@@ -2,8 +2,16 @@
 // 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() {
-  print(dartMath.PI);
+  var pm;
+  try {
+    (pm as Person).firstName = 'Seth'; //LINT
+  } on CastError { }
+
+  Person person = pm;
+  person.firstName = 'Seth';
+}
+
+class Person {
+  var firstName;
 }