CPLAT-12688 Add synthetic Event Utils to master

[joebingham-wk]

To prepare for React 17, there needs to be APIs that abstract the creation of SyntheticEvents so that we can remove dependence on the event class constructors.

Changes

• Add a createSynthetic{eventClass} function for every synthetic event type that allows consumers to create an empty instance, set specific properties on an instance, or merge an instance with new properties
• Add a simple test for every API to ensure that the getters / setters and merge logic is working as expected

QA

• [ ] Review that tests correctly test getting and setting
• [ ] Verify consumer examples of using the APIs work as expected (passing CI should be sufficient)
  • dataset-service-ui
  • graph_ui

[joebingham-wk]

For CI, Dart 2.10.x fails on the formatting step - my understanding is that this was known and ignored for now. Let me know if there is something to do there though!

[sydneyjodon-wk]

QA+1 Everything looks great!

• [x] Review that tests correctly test getting and setting
• [x] Verify consumer examples of using the APIs work as expected (passing CI should be sufficient)
  • [x] dataset-service-ui
  • [x] graph_ui

[joebingham-wk]

Verified CI failed due to that same Dart 2.10.x formatting step!