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This configures polyfills to set up the environment before executing Jest tests. We need to do three things: 1. Set the global `jest` symbol. Jest executing in ESM does not provide the `jest` global and users are expected to import from `@jest/globals` or `import.meta.jest`. Zone.js is not compatible with this yet, so we need to manually define the `jest` global for Zone to read it. 2. Run user polyfills, (typically including `zone.js` and `zone.js/testing`). Zone reads the `jest` global to recognize the environment it is in and patch the relevant functions to load fake async properly. Users can override this part if they are building a Zoneless application or have custom polyfills for other browser functionality. 3. Initalize `TestBed`. This configures the `TestBed` environment so users don't have to manually configure it for each test file. Ordering is very important for these operations, which complicates the implementation somewhat. `zone.js/testing` does not include an import on `zone.js`, meaning there was no guarnatee the bundler would sort their executions in the correct order. Similarly, `zone.js` does not import anything from Jest, so it is not trivial to inject the `globalThis.jest = import.meta.jest;` line before Zone loads. Even setting polyfills to `[jestGlobal, 'zone.js, 'zone.js/testing', initTestBed]` doesn't work because code splitting rearranges the order of operations in an incompatible way. Instead, these are implemented as distinct entry points in `browser-esbuild` with Jest's `--setupFilesAfterEnv` option executing them in the correct order. Ideally, we could drop the global initialization altogether once Zone.js knows to look for `import.meta.jest` in an ESM context. Also we might be able to reduce down to a single polyfills entry point if `zone.js/testing` had an import on `zone.js` to apply correct ordering.