[standalone, mac] Set the process title based on the main script.
[standalone, mac] Set the process title based on the main script.

This makes it easier to identify the responsible program when a Dart process has high CPU or memory usage.

TEST=look at Activity Monitor
Change-Id: Ibcdcc1becf72a25dab5b010accdd8dcd199a0ba1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214137
Reviewed-by: Siva Annamalai <2e618a8cbd7bb6b3d4409061ae407d692a098546@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@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