fix: fix assertion error when change display in input element.

[andycall]

Fix the following assertion error the the examples:

Code

const input = document.querySelector('#input');
document.body.onclick = () => {
   input.style.display = 'inline';
}

ErrorMsg

======== Exception caught by widgets library =======================================================
The following assertion was thrown building WebFContext:
Duplicate keys found.

If multiple keyed widgets exist as children of another widget, they must have unique keys.
WebFRootRenderObjectWidget has multiple children with key [GlobalObjectKey RenderWidget#a2667].
The relevant error-causing widget was: 
  WebFContext WebFContext:file:///Users/andycall.dong/workspace/webf/webf/lib/src/widget/webf.dart:229:14
When the exception was thrown, this was the stack: 
#0      debugChildrenHaveDuplicateKeys.<anonymous closure> (package:flutter/src/widgets/debug.dart:210:7)
#1      debugChildrenHaveDuplicateKeys (package:flutter/src/widgets/debug.dart:217:4)
#2      MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6593:13)
#3      Element.updateChild (package:flutter/src/widgets/framework.dart:3685:15)
#4      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5322:16)
#5      Element.rebuild (package:flutter/src/widgets/framework.dart:5016:7)
#6      ProxyElement.update (package:flutter/src/widgets/framework.dart:5628:5)
#7      Element.updateChild (package:flutter/src/widgets/framework.dart:3685:15)
#8      SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:6441:14)
#9      Element.updateChild (package:flutter/src/widgets/framework.dart:3685:15)
#10     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5322:16)
#11     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5462:11)
#12     Element.rebuild (package:flutter/src/widgets/framework.dart:5016:7)
#13     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2779:19)
#14     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:916:21)
#15     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:360:5)
#16     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1297:15)
#17     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1227:9)
#18     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1085:5)
#19     _invoke (dart:ui/hooks.dart:170:13)
#20     PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:401:5)
#21     _drawFrame (dart:ui/hooks.dart:140:31)