Add TODOs about narrowing JSON catch clauses to the OAuth2 package.

pkg/oauth2/lib/src/handle_access_token_response.dart

Index: pkg/oauth2/lib/src/handle_access_token_response.dart
diff --git a/pkg/oauth2/lib/src/handle_access_token_response.dart b/pkg/oauth2/lib/src/handle_access_token_response.dart
index af94f911f5d34027d8068aaba14f13204484c9b4..cb08d8fd2e3a29b83261838ddc47cb7fd8b03597 100644
--- a/pkg/oauth2/lib/src/handle_access_token_response.dart
+++ b/pkg/oauth2/lib/src/handle_access_token_response.dart
@@ -42,6 +42,7 @@ Credentials handleAccessTokenResponse(
   try {
     parameters = JSON.parse(response.body);
   } catch (e) {
+    // TODO(nweiz): narrow this catch clause once issue 6775 is fixed.
     validate(false, 'invalid JSON');
   }
 
@@ -111,6 +112,7 @@ void _handleErrorResponse(http.Response response, Uri tokenEndpoint) {
   try {
     parameters = JSON.parse(response.body);
   } catch (e) {
+    // TODO(nweiz): narrow this catch clause once issue 6775 is fixed.
     validate(false, 'invalid JSON');
   }