Fixed new lint rules

Author: renefloor

flutter_cache_manager/example/lib/main.dart

@@ -18,8 +18,10 @@ const url = 'https://blurha.sh/assets/images/img1.jpg';
 
 /// Example [Widget] showing the functionalities of flutter_cache_manager
 class CacheManagerPage extends StatefulWidget {
+  const CacheManagerPage({Key key}) : super(key: key);
+
   static ExamplePage createPage() {
-    return ExamplePage(Icons.save_alt, (context) => CacheManagerPage());
+    return ExamplePage(Icons.save_alt, (context) => const CacheManagerPage());
   }
 
   @override
@@ -64,8 +66,10 @@ class _CacheManagerPageState extends State<CacheManagerPage> {
 
   void _removeFile() {
     DefaultCacheManager().removeFile(url).then((value) {
+      //ignore: avoid_print
       print('File removed');
     }).onError((error, stackTrace) {
+      //ignore: avoid_print
       print(error);
     });
     setState(() {

flutter_cache_manager/example/lib/plugin_example/progress_indicator.dart

@@ -14,7 +14,7 @@ class ProgressIndicator extends StatelessWidget {
       child: Row(
         mainAxisAlignment: MainAxisAlignment.center,
         children: <Widget>[
-          Container(
+          SizedBox(
             width: 50.0,
             height: 50.0,
             child: CircularProgressIndicator(

flutter_cache_manager/example/pubspec.yaml

@@ -22,6 +22,7 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+  flutter_lints: ^1.0.4
 
 
 # The following section is specific to Flutter.