Fix all strong mode errors and warnings.

test/extend_test.dart

Index: test/extend_test.dart
diff --git a/test/extend_test.dart b/test/extend_test.dart
index 49c8a9c46169c0bea8ace01e8ed020006bcc723f..afec062ced4bfbee9637f24fe57089df763b8d2e 100644
--- a/test/extend_test.dart
+++ b/test/extend_test.dart
@@ -4,12 +4,13 @@
 
 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());