Get ember-canary passing

[NullVoxPopuli]

Per: emberjs/ember.js#20418

Canary tests were hanging,
Locally:

Died on test #1: Could not find module `@ember/polyfills` imported from `dummy/tests/unit/setup-context-test`
    at TestLoader.moduleLoadFailure (http://localhost:4202/assets/tests.js:1184:22)
    at TestLoader.require (http://localhost:4202/assets/test-support.js:6931:14)
    at TestLoader.loadModules (http://localhost:4202/assets/test-support.js:6923:14)

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To fix, I had to remove @ember/polyfills -- which would be a breaking change -- but that's fine, as we're preparing for v3 anyway