Fix all strong mode errors and warnings.

test/nested_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 nested_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: simpleOptions);
   expect(stylesheet != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());
   expect(prettyPrint(stylesheet), generated);
 }
 
 selectorVariations() {
   final input1 = r'''html { color: red; }''';
   final generated1 = r'''html {
   color: #f00;
@@ skipping 599 matching lines @@
 main() {
   test('Selector and Nested Variations', selectorVariations);
   test('Simple nesting', simpleNest);
   test('Complex nesting', complexNest);
   test('@media nesting', mediaNesting);
   test('Simple &', simpleThis);
   test("Variations &", variationsThis);
   test('Complex &', complexThis);
   test('& with + selector', thisCombinator);
 }