Reduce DDC SyntheticEvent warning noise

[greglittlefield-wf]

Motivation

As a workaround to https://github.com/dart-lang/sdk/issues/43939, our SyntheticEvent classes are non @anonymous.

A consequence of this is that, in the DDC, when event handlers are type-checked in certain situations, they emit console warnings every time.

These warnings can quickly become excessive when interacting with an app.

Solution

• In dev builds of React, wrap console.warn and only emit errors for SyntheticEvent wrappers once
  • Also log an additional message indicating that the warning is expected, and linking to the relevant Dart issue
• Add tests

Boyscouting

• Update outdated name of js_interop_helpers_test test directory and outdated README

Testing

• Verify tests pass in both DDC and dart2js
  • DDC (run pbr test -- -P dartdevc)
  • dart2js (run pbr test -r -- -P dart2js)
• Verify that the tests properly test this behavior
  • Revert compiled JS changes (commit e2fe5a4)
  • Verify that console.warn tests fail in DDC (pbr test -- -P dartdevc) for only the "dev" suites (prod files should not be affected)
• Pull this branch into an app where this issue occurs and verify that the error is only logged once
  • e.g., run the w_context_menu web directory and hover/select items in the context menu

[sydneyjodon-wk]

QA +1

• [x] Verify tests pass in both DDC and dart2js
  • [x] DDC (run pbr test -- -P dartdevc)
  • [x] dart2js (run pbr test -r -- -P dart2js)
• [x] Verify that the tests properly test this behavior
  • [x] Revert compiled JS changes (commit e2fe5a4)
  • [x] Verify that console.warn tests fail in DDC (pbr test -- -P dartdevc) for only the "dev" suites (prod files should not be affected)
• [x] Pull this branch into w_context_menu and w_input_validation and verify that the error is only logged once