title: flutter in android studio says cannot find declaration to go to......i am using the latest version of it v1.7.8+hotfix.4
body: <p>i can't find defination of any of the flutter function or class if i want to see the defination or implementation of <code>FAB</code>(floating action button) it says cannot find the  declaration to go to.
  here you can see the attached files:</p>
  
  <p><a href="https://drive.google.com/open?id=1cgv1pNb43SxZtupl_gAYC0gZrd7CQDy_" rel="nofollow noreferrer">https://drive.google.com/open?id=1cgv1pNb43SxZtupl_gAYC0gZrd7CQDy_</a></p>
  
  <p>i have tried deleting the <code>idea</code> folder and restarting the android studio</p>
  
  <p><code>import 'package:flutter/material.dart';</code></p>
  
  <p>enter code here</p>
  
  <pre><code>class ReusableBox extends StatelessWidget {
    ReusableBox({@required this.colour, this.cardChild, this.onPress});
    final Color colour;
    final Widget cardChild;
    final Function onPress;
    @override
    Widget build(BuildContext context) {
      return GestureDetector(
        onTap: onPress,
        child: Container(
          child: cardChild,
          margin: EdgeInsets.all(15),
          decoration: BoxDecoration(
            color: colour,
            borderRadius: BorderRadius.circular(10),
          ),
        ),
      );
    }
  }
  </code></pre>