Make shelf_web_socket a dev dependency

[brianquinlan]

Works around this issue:

Hey, I'm trying to release package:web_socket_channel:
dependency_overrides:
  shelf_web_socket:
    git:
      ref: master
      url: https://github.com/dart-lang/shelf.git
      path: pkgs/shelf_web_socket
  test: 1.25.2
package:shelf_web_socket:
dependency_overrides:
  test: 1.25.2
And package:test

When I try to publish web_socket_channel, I get a warning that says: 
* Non-dev dependencies are overridden in pubspec.yaml.

  This indicates you are not testing your package against the same versions of its
  dependencies that users will have when they use it.

  This might be necessary for packages with cyclic dependencies.

  Please be extra careful when publishing.
My intent is to force the head version of shelf_web_socket to be used when running the web_socket_channel unit tests.

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