Text: 该widget可以创建一个带格式的文本
用法eg:

 new Text(
   'Hello, $_name! How are you?',
   textAlign: TextAlign.center,
   overflow: TextOverflow.ellipsis,
   style: new TextStyle(fontWeight: FontWeight.bold),
 )

默认构造方法：

const Text(this.data, {
    Key key,
    this.style,
    this.textAlign,
    this.textDirection,
    this.softWrap,
    this.overflow,
    this.textScaleFactor,
    this.maxLines,
  }) : assert(data != null),
       textSpan = null,
       super(key: key);

也就是说，data这个值肯定是不能为null，必须要传的，textSpan设为null
其他构造方法：

const Text.rich(this.textSpan, {
    Key key,
    this.style,
    this.textAlign,
    this.textDirection,
    this.softWrap,
    this.overflow,
    this.textScaleFactor,
    this.maxLines,
  }): assert(textSpan != null),
      data = null,
      super(key: key);

其实Text的最终build结果为一个RichText对象：

@override
  Widget build(BuildContext context) {
    final DefaultTextStyle defaultTextStyle = DefaultTextStyle.of(context);
    TextStyle effectiveTextStyle = style;
    if (style == null || style.inherit)
      effectiveTextStyle = defaultTextStyle.style.merge(style);
    return new RichText(
      textAlign: textAlign ?? defaultTextStyle.textAlign ?? TextAlign.start,
      textDirection: textDirection, // RichText uses Directionality.of to obtain a default if this is null.
      softWrap: softWrap ?? defaultTextStyle.softWrap,
      overflow: overflow ?? defaultTextStyle.overflow,
      textScaleFactor: textScaleFactor ?? MediaQuery.textScaleFactorOf(context),
      maxLines: maxLines ?? defaultTextStyle.maxLines,
      text: new TextSpan(
        style: effectiveTextStyle,
        text: data,
        children: textSpan != null ? <TextSpan>[textSpan] : null,
      ),
    );
  }

而RichText则是：

const RichText({
    Key key,
    @required this.text,
    this.textAlign: TextAlign.start,
    this.textDirection,
    this.softWrap: true,
    this.overflow: TextOverflow.clip,
    this.textScaleFactor: 1.0,
    this.maxLines,
  }) : assert(text != null),
       assert(textAlign != null),
       assert(softWrap != null),
       assert(overflow != null),
       assert(textScaleFactor != null),
       assert(maxLines == null || maxLines > 0),
       super(key: key);