[Presubmit] Fixes presubmit for breaking API change.

The API change happened in:
https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2079411

Change-Id: I1c826f102c1366c193a75fb5e17a355dad34cbc3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/137977
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Clement Skau <cskau@google.com>

Author: Clement Skau

runtime/PRESUBMIT.py

@@ -59,10 +59,11 @@ def CheckFormatted(input_api, output_api):
     def convert_warning_to_error(presubmit_result):
         if not presubmit_result.fatal:
             # Convert this warning to an error.
-            stream = StringIO.StringIO()
-            presubmit_result.handle(stream)
-            message = stream.getvalue()
-            return output_api.PresubmitError(message)
+            result_json = presubmit_result.json_format()
+            return output_api.PresubmitError(
+                message=result_json['message'],
+                items=result_json['items'],
+                long_text=result_json['long_text'])
         return presubmit_result
 
     results = input_api.canned_checks.CheckPatchFormatted(input_api, output_api)