The number of test failures / flakes during PR and push builds on Travis due to dirty global state appears to have increased over the recent weeks. It's time to explore ideas that will reduce flakiness by isolating tests from one another and eliminating dirty global state.

Some ideas:

• Update beforeEach and afterEach in test/_init_tests.js to be more thorough with how they clear state between tests
• Prevent tests from performing operations that modify global state
• Fail tests that when errors are logged by the runtime (unless they're expected) (🏗 Surface console errors during tests as mocha failures #14336)
• Create a new window object (with a new this) for each test (inefficient? infeasible?)

(More ideas welcome via comments below)