Fix all strong mode errors and warnings.

example/call_parser.dart

Index: example/call_parser.dart
diff --git a/example/call_parser.dart b/example/call_parser.dart
index 7179a170f83697017689de04253dfa474588416d..3ae588932053d8be91f26d9589f385d1de4cfca6 100644
--- a/example/call_parser.dart
+++ b/example/call_parser.dart
@@ -2,6 +2,7 @@
 // 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(
@@ -16,7 +17,7 @@ const _default = const css.PreprocessorOptions(
  * 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);
 }
@@ -27,7 +28,7 @@ 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:');