Allow instrumentation of transformed files with weird file extensions

The default configuration of babel-plugin-istanbul only allows
instrumentation of files with extensions .js, .cjs, .mjs, .ts, .tsx,
.jsx (https://github.com/istanbuljs/schema/blob/v0.1.2/index.js#L71).
However, we know that we’re running it on code that we just
transformed to JavaScript, so the source language no longer matters,
and we should disable this check to get complete instrumentation.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

CHANGELOG.md

@@ -27,6 +27,7 @@
 - `[jest-snapshot]` Downgrade semver to v6 to support node 8 ([#9451](https://github.com/facebook/jest/pull/9451))
 - `[jest-snapshot]` Properly indent new snapshots in the presences of existing ones ([#9523](https://github.com/facebook/jest/pull/9523))
 - `[jest-transform]` Correct sourcemap behavior for transformed and instrumented code ([#9460](https://github.com/facebook/jest/pull/9460))
+- `[jest-transform]` Allow instrumentation of transformed files with weird file extensions ([#9589](https://github.com/facebook/jest/pull/9589))
 - `[pretty-format]` Export `OldPlugin` type ([#9491](https://github.com/facebook/jest/pull/9491))
 
 ### Chore & Maintenance

packages/jest-transform/src/ScriptTransformer.ts

@@ -213,6 +213,7 @@ export default class ScriptTransformer {
             // files outside `cwd` will not be instrumented
             cwd: this._config.rootDir,
             exclude: [],
+            extension: false,
             // Needed for correct coverage as soon as we start storing a source map of the instrumented code
             inputSourceMap: input.map,
             useInlineSourceMaps: false,