Flutter Image.asset Mapping valu

Image控件即为图片控件。
Image控件有多种构造函数：

new Image，用于从ImageProvider获取图像。
new Image.network，用于从URL地址获取图像。
new Image.file，用于从File获取图像。
new Image.asset，用于使用key从AssetBundle获取图像。
上面三个很简单，第四个在使用的时候，一不小心就会报如下错误

Error on line 44, column 10 of pubspec.yaml: Mapping values are not allowed here. Did you miss a colon earlier?
assets:
^
Unable to reload your application because "flutter packages get" failed to update package dependencies.
Exception: pub get failed (65)

Image.asset 使用之前需要在pubspec.yaml 中将资源文件标识出来

flutter:

  # The following line ensures that the Material Icons font is
  # included with your application, so that you can use the icons in
  # the material Icons class.
  uses-material-design: true

   assets:
    - lib/image/


  # To add assets to your application, add an assets section, like this:
  # assets:
  #  - images/a_dot_burr.jpeg
  #  - images/a_dot_ham.jpeg

报错信息很朦胧，实际原因是： assets: 前面多了个空格 ，。。。。。