Stop working around issue 6775.

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 5e5f6e279f59b321f74843fde6833d61fc067497..c14b8d1e2a697fff3d5a87ce11a8c974bd812d75 100644
--- a/pkg/oauth2/lib/src/handle_access_token_response.dart
+++ b/pkg/oauth2/lib/src/handle_access_token_response.dart
@@ -40,8 +40,7 @@ Credentials handleAccessTokenResponse(
   var parameters;
   try {
     parameters = JSON.parse(response.body);
-  } catch (e) {
-    // TODO(nweiz): narrow this catch clause once issue 6775 is fixed.
+  } on FormatException catch (e) {
     validate(false, 'invalid JSON');
   }
 
@@ -110,8 +109,7 @@ void _handleErrorResponse(http.Response response, Uri tokenEndpoint) {
   var parameters;
   try {
     parameters = JSON.parse(response.body);
-  } catch (e) {
-    // TODO(nweiz): narrow this catch clause once issue 6775 is fixed.
+  } on FormatException catch (e) {
     validate(false, 'invalid JSON');
   }