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

lib/src/rules.dart

Index: lib/src/rules.dart
diff --git a/lib/src/rules.dart b/lib/src/rules.dart
index 14b8ba6d632bbf85e761095129950fb6c091785c..34264282c2d0da4f6a14c1be112fa4bafc3e9c07 100644
--- a/lib/src/rules.dart
+++ b/lib/src/rules.dart
@@ -10,6 +10,7 @@ import 'package:linter/src/config.dart';
 import 'package:linter/src/linter.dart';
 import 'package:linter/src/rules/always_declare_return_types.dart';
 import 'package:linter/src/rules/always_specify_types.dart';
+import 'package:linter/src/rules/avoid_as.dart';
 import 'package:linter/src/rules/camel_case_types.dart';
 import 'package:linter/src/rules/constant_identifier_names.dart';
 import 'package:linter/src/rules/empty_constructor_bodies.dart';
@@ -32,6 +33,7 @@ import 'package:linter/src/rules/unnecessary_getters_setters.dart';
 final Registry ruleRegistry = new Registry()
   ..register(new AlwaysDeclareReturnTypes())
   ..register(new AlwaysSpecifyTypes())
+  ..register(new AvoidAs())
   ..register(new CamelCaseTypes())
   ..register(new ConstantIdentifierNames())
   ..register(new EmptyConstructorBodies())