title: Target URI doesn't exist, when trying to import a package in flutter
body: <p>I have stumbled upon a problem in importing the package in Flutter, I tried to solve this by running flutter packages get and also shutting down the project in Android studio and reopening it. </p>
  
  <pre><code>import 'package:task_02_category_widget/category.dart';
  </code></pre>
  
  <p>Here is the line above, and the error I'm running into when I run it gives the following error in the console. </p>
  
  <pre><code>Your application could not be compiled, because its dependencies could 
  not be established.
  The following Dart file:
    /Users/username/Documents/flutter_rectangle_2/lib/main.dart
  ...refers, in an import, to the following library:
  package:task_02_category_widget/category.dart
  That library is in a package that is not known. Maybe you forgot to 
  mention it in your pubspec.yaml file?
  </code></pre>