Mark oauth2 as running outside of dart:io contexts.

pkg/oauth2/README.md

 A client library for authenticating with a remote service via OAuth2 on
 behalf of a user, and making authorized HTTP requests with the user's OAuth2
-credentials. Currently this only works where `dart:io` is available.
+credentials.
 
 OAuth2 allows a client (the program using this library) to access and
 manipulate a resource that's owned by a resource owner (the end user) and
 lives on a remote server. The client directs the resource owner to an
 authorization server (usually but not always the same as the server that
 hosts the resource), where the resource owner tells the authorization server
 to give the client an access token. This token serves as proof that the
 client has permission to access resources on behalf of the resource owner.
 
 OAuth2 provides several different methods for the client to obtain
@@ skipping 77 matching lines @@
     // Once we're done with the client, save the credentials file. This
     // ensures that if the credentials were automatically refreshed
     // while using the client, the new credentials are available for the
     // next run of the program.
     return credentialsFile.open(FileMode.WRITE).then((file) {
       return file.writeString(client.credentials.toJson());
     }).then((file) => file.close()).then((_) => result);
   });
 }).then(print);
 ```