Fix all strong mode errors and warnings.

test/big_1_test.dart

Index: test/big_1_test.dart
diff --git a/test/big_1_test.dart b/test/big_1_test.dart
index f92ea82ec0d115c60410d5a7cf350b13f0f4641b..9a6a584c01548d67ed10ca8c8443249319b8da71 100644
--- a/test/big_1_test.dart
+++ b/test/big_1_test.dart
@@ -4,11 +4,12 @@
 
 library big_1_test;
 
+import 'package:csslib/src/messages.dart';
 import 'package:test/test.dart';
 import 'testing.dart';
 
 compilePolyfillAndValidate(String input, String generated) {
-  var errors = [];
+  var errors = <Message>[];
   var stylesheet = polyFillCompileCss(input, errors: errors, opts: options);
   expect(stylesheet != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());