Fix all strong mode errors and warnings.

test/var_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 var_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());
   expect(prettyPrint(stylesheet), generated);
 }
 
 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());
   expect(prettyPrint(stylesheet), generated);
 }
 
 void simpleVar() {
   final input = ''':root {
   var-color-background: red;
   var-color-foreground: blue;
@@ skipping 302 matching lines @@
   background: #0a0 url("test.png") no-repeat right top;
 }
 .test-6 {
   border: #f00 solid 20px;
 }''';
 
   compilePolyfillAndValidate(input, generatedPolyfill);
 }
 
 void undefinedVars() {
-  final errors = [];
+  final errors = <Message>[];
   final input = ''':root {
   var-color-background: red;
   var-color-foreground: blue;
 
   var-a: var(b);
   var-b: var(c);
   var-c: #00ff00;
 
   var-one: var(two);
   var-two: var(one);
@@ skipping 261 matching lines @@
   src: local(Gentium Bold), local(Gentium-Bold), url("GentiumBold.ttf");
   unicode-range: U+0A-FF, U+980-9FF, U+????, U+3???;
 }
 .foobar {
   grid-columns: 10px ("content" 1fr 10px) [4];
 }''';
   compilePolyfillAndValidate(input, generatedPolyfill);
 }
 
 testVar() {
-  final errors = [];
+  final errors = <Message>[];
   final input = '''
 @color-background: red;
 @color-foreground: blue;
 
 .test {
   background-color: var(color-background);
   color: var(color-foreground);
 }
 ''';
   final generated = '''
@@ skipping 33 matching lines @@
   stylesheet = parseCss(input, errors: errors..clear(), opts: simpleOptions);
 
   expect(stylesheet != null, true);
   expect(errors.isEmpty, true, reason: errors.toString());
   expect(prettyPrint(stylesheet), generated2);
 
   compileAndValidate(input2, generated2);
 }
 
 testLess() {
-  final errors = [];
+  final errors = <Message>[];
   final input = '''
 @color-background: red;
 @color-foreground: blue;
 
 .test {
   background-color: var(color-background);
   color: var(color-foreground);
 }
 ''';
   final generated = '''var-color-background: #f00;
@@ skipping 75 matching lines @@
   background-color: #fff;
   color: #fff;
   border-color: #fff;
 }
 .test-1 {
   color: #000;
 }''');
 }
 
 void includes() {
-  var errors = [];
+  var errors = <Message>[];
   var file1Input = r'''
 :root {
   var-redef: #0f0;
 
   var-a1: #fff;
   var-a2: var(a1);
   var-a3: var(a2);
 
   var-redef: #000;
 }
@@ skipping 129 matching lines @@
   test('Expressions var', expressionsVar);
   test('Default value in var()', defaultVar);
   test('CSS Parser only var', parserVar);
   test('Var syntax', testVar);
   test('Indirects', testIndirects);
   test('Forward Refs', undefinedVars);
   test('Less syntax', testLess);
   test('Polyfill', polyfill);
   test('Multi-file', includes);
 }