Fix all strong mode errors and warnings.

test/extend_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.
 
 library extend_test;
 
+import 'package:csslib/src/messages.dart';
 import 'package:test/test.dart';
 
 import 'testing.dart';
 
 compileAndValidate(String input, String generated) {
-  var errors = [];
+  var errors = <Message>[];
   var stylesheet = compileCss(input, errors: errors, opts: options);
   expect(stylesheet != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());
   expect(prettyPrint(stylesheet), generated);
 }
 
 void simpleExtend() {
   compileAndValidate(r'''
 .error {
   border: 1px red;
@@ skipping 202 matching lines @@
 
 main() {
   test("Simple Extend", simpleExtend);
   test("complex", complexSelectors);
   test("multiple", multipleExtends);
   test("chaining", chaining);
   test("nested selectors", nestedSelectors);
   test("nested many selector sequences", nestedMulty);
   test("N-way extends", nWayExtends);
 }