Fix all strong mode errors and warnings.

test/visitor_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 visitor_test;
 
+import 'package:csslib/src/messages.dart';
 import 'package:csslib/visitor.dart';
 import 'package:test/test.dart';
 
 import 'testing.dart';
 
 class ClassVisitor extends Visitor {
   final List expectedClasses;
   final Set<String> foundClasses = new Set();
 
   ClassVisitor(this.expectedClasses);
 
   void visitClassSelector(ClassSelector node) {
     foundClasses.add(node.name);
   }
 
   bool get matches {
     bool match = true;
     foundClasses.forEach((value) {
       match = match && expectedClasses.contains(value);
     });
     expectedClasses.forEach((value) {
       match = match && foundClasses.contains(value);
     });
 
     return match;
   }
 }
 
 void testClassVisitors() {
-  var errors = [];
+  var errors = <Message>[];
   var in1 = '.foobar { }';
 
   var s = parseCss(in1, errors: errors);
 
   expect(s != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());
 
   var clsVisits = new ClassVisitor(['foobar'])..visitTree(s);
   expect(clsVisits.matches, true);
 
@@ skipping 30 matching lines @@
 
   void visitClassSelector(ClassSelector node) {
     emit('.${_prefix}_${node.name}');
   }
 }
 
 String polyfillPrint(String prefix, StyleSheet ss) =>
     (new PolyfillEmitter(prefix)..visitTree(ss, pretty: true)).toString();
 
 void testPolyFill() {
-  var errors = [];
+  var errors = <Message>[];
   final input = r'''
 .foobar { }
 div.xyzzy { }
 #foo .foo .bar .foobar { }
 ''';
 
   final generated = r'''
 .myComponent_foobar {
 }
 div.myComponent_xyzzy {
 }
 #foo .myComponent_foo .myComponent_bar .myComponent_foobar {
 }''';
 
   var s = parseCss(input, errors: errors);
   expect(s != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());
 
   final emitted = polyfillPrint('myComponent', s);
   expect(emitted, generated);
 }
 
 main() {
   test('Class Visitors', testClassVisitors);
   test('Polyfill', testPolyFill);
 }