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

test/rule_test.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.
 
 library linter.test.rule;
 
 import 'dart:io';
 
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 323 matching lines @@
 AnnotationMatcher matchesAnnotation(
         String message, ErrorType type, int lineNumber) =>
     new AnnotationMatcher(new Annotation(message, type, lineNumber));
 
 testEach(Iterable<String> values, dynamic f(String s), Matcher m) {
   values.forEach((s) => test('"$s"', () => expect(f(s), m)));
 }
 
 testRule(String ruleName, File file, {bool debug: false}) {
   test('$ruleName', () {
+    if (!file.existsSync()) {
+      throw new Exception('No rule found defined at: ${file.path}');
+    }
+
     var expected = <AnnotationMatcher>[];
 
     int lineNumber = 1;
     for (var line in file.readAsLinesSync()) {
       var annotation = extractAnnotation(line);
       if (annotation != null) {
         annotation.lineNumber = lineNumber;
         expected.add(new AnnotationMatcher(annotation));
       }
       ++lineNumber;
     }
 
-    LinterOptions options = new LinterOptions(
-        [ruleRegistry[ruleName]].where((rule) => rule != null))
+    LintRule rule = ruleRegistry[ruleName];
+    if (rule == null) {
+      print('WARNING: Test skipped -- rule `$ruleName` is not registered.');
+      return;
+    }
+
+    LinterOptions options = new LinterOptions([rule])
       ..useMockSdk = true
       ..packageRootPath = '.';
 
     DartLinter driver = new DartLinter(options);
 
     Iterable<AnalysisErrorInfo> lints = driver.lintFiles([file]);
 
     List<Annotation> actual = [];
     lints.forEach((AnalysisErrorInfo info) {
       info.errors.forEach((AnalysisError error) {
@@ skipping 83 matching lines @@
 
 class NoFilter implements LintFilter {
   @override
   bool filter(AnalysisError lint) => false;
 }
 
 class ResultReporter extends DetailedReporter {
   ResultReporter(Iterable<AnalysisErrorInfo> errors)
       : super(errors, new NoFilter(), stdout);
 }