flutter 报错之 No MediaQuery widget found.

这里通过一个HelloWorld app来展示如何创建一个Widget并展示出来，并区分Material和非Material环境。
hello_word.dart里的代码如下：  MaterialApp

class HelloWorld {
  static Widget helloWorld() {
    return new MaterialApp(
      title: 'Hello World',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new Scaffold(
        appBar: new AppBar(title: new Text('Hello World')),
        body: new Center(
            child: new Text(
          "Hello World",
          style: new TextStyle(fontSize: 32.0),
        )),
      ),
    );
  }

  static Widget hellWorldWithoutMaterial() {
    return new Container(
      decoration: new BoxDecoration(color: Colors.white),
      child: new Center(
        child: new Text(
          'Hello World',
          textDirection: TextDirection.ltr, // 这一句必须有
          style: new TextStyle(color: Colors.black, fontSize: 40.0),
        ),
      ),
    );
  }
}

而在展示上，主要的区别在于非Material下，没有Appbar、背景色和标题等，所有的内容都需要自定义。

注意⚠️：

1、Scaffold必须放在MaterialApp里面，否则会报错，大致如下：

══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown building Scaffold(dirty, state: ScaffoldState#6f74d):
No MediaQuery widget found.
Scaffold widgets require a MediaQuery widget ancestor.
The specific widget that could not find a MediaQuery ancestor was:
Scaffold
The ownership chain for the affected widget is:
Scaffold ← MyApp ← [root]
Typically, the MediaQuery widget is introduced by the MaterialApp or WidgetsApp widget at the top of
your application widget tree.