[ dart:io ] Fix format of HTTP body events logged to timeline
[ dart:io ] Fix format of HTTP body events logged to timeline

Related to an issue documented here: https://github.com/flutter/devtools/issues/2335.

Change-Id: I4d15da9f897ed2a5dd4253c1ca8c8636658555c9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/163127
Reviewed-by: Kenzie Schmoll <7294156c507c19d848723c5abf67fe40039705bb@google.com>
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Ben Konyi <67ffcaedcc3dc3d87dddb9627b72dd41756b31b1@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk