Add support for SharedArrayBuffers in Dart testing

[srujzs]

Specifically, we would need to support the following headers (we're already running in a secure context, so we just need to be cross-origin isolated):

Cross-Origin-Opener-Policy
Cross-Origin-Embedder-Policy

https://developer.mozilla.org/en-US/docs/Web/API/Window/crossOriginIsolated

These are the prerequisites to use SharedArrayBuffers: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer#security_requirements

I did an initial attempt by adding these headers here:

sdk/pkg/test_runner/lib/src/testing_servers.dart

Line 352 in c30dd25

response.headers.set("Access-Control-Allow-Origin", "*");

, but this makes the test driver timeout. The test still runs, but the driver never completes. This is likely because Cross-Origin-Opener-Policy: same-origin means that the window's opener is always null:

sdk/pkg/test_runner/lib/src/test_controller.js

Line 144 in 7d1998d

if (window != window.parent) {

, so we can never tell the driver the test has completed. Maybe we should have the driver listen for messages on the opened window instead of listening for messages on itself?