Import Flutter module doesn't work on Windows

[mit-mit]

1. Follow the steps on https://github.com/flutter/flutter/wiki/Add-Flutter-to-existing-apps to import a Flutter module to an Android app using Windows.

=> Gradle complains that a character is incorectly escaped:

settings file 'C:\Users\mit\tmp\plugtest\flutter_plugin\example\MyApplication\settings.gradle': 5: unexpected char: '\' @ line 5, column 6.
     '..\..\..\mod42\.android\include_flutter.groovy'
        ^

Changing the string to this seems to fix it:

evaluate(new File(
  settingsDir.parentFile,
  '..\\..\\..\\mod42\\.android\\include_flutter.groovy'
))

[mit-mit]

cc @stevemessick