Unnecessary new (#20138)

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests
diff --git a/dev/tools/java_and_objc_doc.dart b/dev/tools/java_and_objc_doc.dart
index 52a5acb..8248265 100644
--- a/dev/tools/java_and_objc_doc.dart
+++ b/dev/tools/java_and_objc_doc.dart
@@ -13,7 +13,7 @@
 /// This script downloads an archive of Javadoc and objc doc for the engine from
 /// the artifact store and extracts them to the location used for Dartdoc.
 Future<Null> main(List<String> args) async {
-  final String engineVersion = new File('bin/internal/engine.version').readAsStringSync().trim();
+  final String engineVersion = File('bin/internal/engine.version').readAsStringSync().trim();
 
   final String javadocUrl = 'https://storage.googleapis.com/flutter_infra/flutter/$engineVersion/android-javadoc.zip';
   generateDocs(javadocUrl, 'javadoc', 'io/flutter/view/FlutterView.html');
@@ -25,21 +25,21 @@
 Future<Null> generateDocs(String url, String docName, String checkFile) async {
   final http.Response response = await http.get(url);
 
-  final Archive archive = new ZipDecoder().decodeBytes(response.bodyBytes);
+  final Archive archive = ZipDecoder().decodeBytes(response.bodyBytes);
 
-  final Directory output = new Directory('$kDocRoot/$docName');
+  final Directory output = Directory('$kDocRoot/$docName');
   print('Extracting $docName to ${output.path}');
   output.createSync(recursive: true);
 
   for (ArchiveFile af in archive) {
     if (af.isFile) {
-      final File file = new File('${output.path}/${af.name}');
+      final File file = File('${output.path}/${af.name}');
       file.createSync(recursive: true);
       file.writeAsBytesSync(af.content);
     }
   }
 
-  final File testFile = new File('${output.path}/$checkFile');
+  final File testFile = File('${output.path}/$checkFile');
   if (!testFile.existsSync()) {
     print('Expected file ${testFile.path} not found');
     exit(1);