Allow unrestricted Promise usage in testharness.js

rfcs/promise.md

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+# RFC 93: Allow testharness.js to depend on Promise being available
+
+## Summary
+
+Currently testharness.js has a test-enforced policy that ES Promises
+are only required by features which specifically depend on promises
+(e.g. `promise_test`). Other functionality must work in the absence of
+promises.
+
+This RFC removes that restriction, allowing ES Promises to be used
+throughout the testharness.js codebase.
+
+## Details
+
+Historically ES Promises were not widely supported across browser
+engines. Native promises became available in 2014, and even some years
+later browsers that were known to make use of web-platform-tests were
+running JavaScript engine versions without Promise support. In
+particular Servo was making heavy use of web-platform-tests but didn't
+support promises until late
+[2016](https://github.com/servo/servo/issues/4282), whilst Internet
+Explorer has never had Promise support.
+
+Due to the Servo requirements in particular, the testharness.js tests
+were written to run in a mode with the [global promise constructor
+removed](https://github.com/web-platform-tests/wpt/commit/08995880734011eed359c4e79420db29626016ff#diff-d33a74992c12a398b4876be7ea865df0387f779f740be2ff79e792f7d4db5badR12-R14). This
+ensures that features which don't explicitly depend on Promise don't
+accidentally rely on it, allowing most tests to run in
+implementations without Promises.
+
+Today the number of browsers which might conceivably run wpt but lack
+Promise support is very low. Many new DOM APIs require promises, so
+the only implementations which lack Promise support are those such as
+IE 11 which are no longer receiving feature updates. This makes it
+safe to rely on promises across the entire testharness codebase. It
+opens up the possibility of refactorings to make the code simpler, and
+removes a point of confusion for new contributors who are unaware of
+the unusual policy of avoiding promises.
+
+The proposal is that we allow promises throughout testharness as a
+matter of policy, and remove the variants feature from the
+testharness.js tests, along with all usage of it. Since this feature
+is currently only used for with and without promise variants, we can
+simplify test authoring by not having the additional complexity.
+
+## Risks
+
+* Some unusual older browsers might still be running wpt. For example
+  we know that the WAVE server is used for testing TV-based browsers,
+  which could conceivably be so outdated as to not support
+  promises. We don't have much insight into how this is used so could
+  conceivably result in "dark matter" breakage.
+
+* Removing the variants feature from testharness.js tests entirely
+  might be premature if we find we need similar functionality in the
+  future. However the fact that we've currently only used it for this
+  one use case, and the fact that it adds noticeable complexity to test
+  authoring and to the harness suggests we should consider re-adding
+  the feature if we find an additional use case.