Dart / Flutter - Validating a string for URL

I'm trying to create a system in which I can validate a string to check if it's a url or not. (https://www.google.com/)

I found the flutter package validator but this isn't dart 2 compatible, so won't be compatible with my code.

Similarly named, I also found the validators package but I can't seem to get it working correctly with my code, as it throws the following error; (hope you like my project name ;)

Because every version of flutter_test from sdk depends on test 1.3.0 
and every version of validators depends on test ^1.3.3, flutter_test from sdk is incompatible with validators.
So, because gucci depends on both validators ^1.0.0+1 and flutter_test any from sdk, version solving failed.
Unable to reload your application because "flutter packages get" failed to update package dependencies.
Exception: pub get failed (1)

If you could either find a fix for validators so that it doesn't throw this error and thus works correctly with my code, or suggest another method of validating a string to be URL compliant that would be great.

Thanks

Edit - My pubspec.yaml file

name: gucci
description: A new Flutter project.

dependencies:
  flutter:
    sdk: flutter

  cupertino_icons:
  barcode_scan:
  gradient_app_bar:
  url_launcher:
  validate: ^1.7.0

dev_dependencies:
  flutter_test:
    sdk: flutter

  fonts:
    - family: PlayfairDisplay
      fonts:
        - asset: fonts/PlayfairDisplay-BoldItalic.ttf

    - family: Kanit
      fonts:
        - asset: fonts/Kanit-ExtraBoldItalic.ttf

    - family: Poppins
      fonts:
        - asset: fonts/Poppins-BoldItalic.ttf

    - family: PoppinsLightItalic
      fonts:
        - asset: fonts/Poppins-LightItalic.ttf

    - family: PoppinsMediumItalic
      fonts:
        - asset: fonts/Poppins-MediumItalic.ttf

Solution

For some reason, the validators package is requiring a pretty recent version of flutter's testing library rather than leaving it up to the application. Maybe there's a good reason for that (i.e. they're using a new feature).

The flutter engine internally requires a particular version of the flutter_test library (which is why it's generally a bad idea to specify a version of it). So to fix this you'll have to upgrade your flutter by running flutter upgrade. If you're already at the most recent version of the channel you're in, you may have to run flutter channel dev or flutter channel master to switch to a channel (branch really) that's updated more often.

I run on the dev branch/channel for the most part and while it very occasionally has problems, it doesn't happen a lot. I'd advise against using the master branch if possible though.