Add utility methods for sending non-form data in pkg/http.

pkg/oauth2/lib/src/credentials.dart

Index: pkg/oauth2/lib/src/credentials.dart
diff --git a/pkg/oauth2/lib/src/credentials.dart b/pkg/oauth2/lib/src/credentials.dart
index 13d33204cc2164a32a0b237e19c92c4c1f386119..ce4e06454ffd52a59c82f24b845ce7a347fbdd2d 100644
--- a/pkg/oauth2/lib/src/credentials.dart
+++ b/pkg/oauth2/lib/src/credentials.dart
@@ -173,7 +173,7 @@ class Credentials {
       };
       if (!scopes.isEmpty) fields["scope"] = scopes.join(' ');
 
-      return httpClient.post(tokenEndpoint, fields: fields);
+      return httpClient.post(tokenEndpoint, body: fields);
     }).then((response) {
       return handleAccessTokenResponse(
           response, tokenEndpoint, startTime, scopes);