[frontend_server] allow dart define to contain commas
[frontend_server] allow dart define to contain commas

package:args seems to chop off commas unless disabled. This prevents users from having
dart defines that contain commas.

Bug: https://github.com/flutter/flutter/issues/72105
Change-Id: I9900a0db0d0ad24348dd06acff7a48d3c579564f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/178881
Reviewed-by: Alexander Aprelev <dfb74c35a357c2059971a4bf7fe26b6a7c18085d@google.com>
Commit-Queue: Jonah Williams <07430f204b9a104051fbdce3c555ab8df64d2479@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