Fix all strong mode errors and warnings.

example/call_parser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 
 import 'package:csslib/parser.dart' as css;
+import 'package:csslib/src/messages.dart';
 import 'package:csslib/visitor.dart';
 
 const _default = const css.PreprocessorOptions(
     useColors: false,
     checked: true,
     warningsAsErrors: true,
     inputFile: 'memory');
 
 /**
  * Spin-up CSS parser in checked mode to detect any problematic CSS.  Normally,
  * CSS will allow any property/value pairs regardless of validity; all of our
  * tests (by default) will ensure that the CSS is really valid.
  */
 StyleSheet parseCss(String cssInput,
-    {List errors, css.PreprocessorOptions opts}) {
+    {List<Message> errors, css.PreprocessorOptions opts}) {
   return css.parse(cssInput,
       errors: errors, options: opts == null ? _default : opts);
 }
 
 // Pretty printer for CSS.
 var emitCss = new CssPrinter();
 String prettyPrint(StyleSheet ss) =>
     (emitCss..visitTree(ss, pretty: true)).toString();
 
 main() {
-  var errors = [];
+  var errors = <Message>[];
 
   // Parse a simple stylesheet.
   print('1. Good CSS, parsed CSS emitted:');
   print('   =============================');
   var stylesheet = parseCss(
       '@import "support/at-charset-019.css"; div { color: red; }'
       'button[type] { background-color: red; }'
       '.foo { '
       'color: red; left: 20px; top: 20px; width: 100px; height:200px'
       '}'
@@ skipping 46 matching lines @@
   var selectorAst = css.selector('#div .foo', errors: errors);
   if (!errors.isEmpty) {
     print("Got ${errors.length} errors.\n");
     for (var error in errors) {
       print(error);
     }
   } else {
     print(prettyPrint(selectorAst));
   }
 }