Make Flutter repo Dart SDK 2.1-dev3.1 compatible (#21853)

This is a blocker for Google roll since we are not at dev4.0:
- Future is not yet part of dart:core.
- Future.sync().then<dynamic>... causes failure without the new keyword.

packages/flutter/lib/src/rendering/editable.dart

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+//ignore: Remove this once Google catches up with dev.4 Dart.
+import 'dart:async';
 import 'dart:math' as math;
 import 'dart:ui' as ui show TextBox;
 

packages/flutter_tools/lib/src/commands/daemon.dart

@@ -184,7 +184,9 @@ abstract class Domain {
   String toString() => name;
 
   void handleCommand(String command, dynamic id, Map<String, dynamic> args) {
-    Future<dynamic>.sync(() {
+    // Remove 'new' once Google catches up to dev4.0 Dart SDK.
+    //ignore: unnecessary_new
+    new Future<dynamic>.sync(() {
       if (_handlers.containsKey(command))
         return _handlers[command](args);
       throw 'command not understood: $name.$command';