title: Error: Don't import implementation files from another package
body: <p>I am trying to make a custom stepper in Flutter. I copied the implementation of stepper.dart, save my file to my own lib folder and fixed the imports to remove the errors. </p>
  
  <pre><code>import 'package:flutter/src/material/button_theme.dart';
  import 'package:flutter/src/material/colors.dart';
  import 'package:flutter/src/material/debug.dart';
  import 'package:flutter/src/material/flat_button.dart';
  import 'package:flutter/src/material/icons.dart';
  import 'package:flutter/src/material/ink_well.dart';
  import 'package:flutter/src/material/material.dart';
  import 'package:flutter/src/material/material_localizations.dart';
  import 'package:flutter/src/material/theme.dart';
  import 'package:flutter/src/material/typography.dart';
  </code></pre>
  
  <p>Errors are removed but dart says that "Don't import implementation files from another package." </p>
  
  <p>May I know if it's safe to proceed? OR is there another way to implement custom widgets? Or should I transfer the location of my custom stepper file? Thanks.</p>