Delete fail.dart (#6699)

I think we must have accidentally checked this in some time.

examples/flutter_gallery/lib/demo/fail.dart

-
-import 'package:flutter/material.dart';
-
-class LimitDemo extends StatelessWidget {
-  @override
-  Widget build(BuildContext context) {
-    return new Scaffold(
-      appBar: new AppBar(
-        title: new Text('LimitedBox test')
-      ),
-      body: new Block(
-        children: <Widget>[
-          new LimitedBox(
-            maxWidth: 100.0,
-            maxHeight: 100.0,
-            child: new Container(
-              decoration: new BoxDecoration(backgroundColor: const Color(0xFF00FF00))
-            )
-          )
-        ]
-      )
-    );
-  }
-}