Move Dart licenses into a separate golden file

This will avoid conflicts between Dart rolls and rolls of other third party
components such as ANGLE.

Author: jason-simmons

ci/licenses_golden/excluded_files

@@ -1480,6 +1480,7 @@
 ../../../third_party/colorama/src/requirements.txt
 ../../../third_party/colorama/src/screenshots
 ../../../third_party/colorama/src/setup.py
+../../../third_party/dart
 ../../../third_party/dart/.clang-format
 ../../../third_party/dart/.dart_tool
 ../../../third_party/dart/.git