The issue is probably old Dart SDKs that are in your path. Check your path and remove them.
Originally answered on the GitHub issue by Devin Carew:
If you're running pub global activate directly, it's using the version
of the dart sdk that's on your path, which may be different than the
version shipped with flutter (you can run dart --version to see the
version on your path). I suspect that you have an old version of dart,
and pub is having trouble resolving a version of package:http that
will work with it. You'll probably want to update your path to include
path/to/flutter/bin/cache/dart-sdk/bin, or could try running flutter packages pub global activate flutter_plugin_tools.