chore: migrate babel-jest to TypeScript

[SimenB]

Summary

Built diff:

diff --git c/packages/babel-jest/build/index.js w/packages/babel-jest/build/index.js
index 09b426d20..fc3dd7f8d 100644
--- c/packages/babel-jest/build/index.js
+++ w/packages/babel-jest/build/index.js
@@ -102,8 +102,9 @@ const jestPresetPath = require.resolve('babel-preset-jest');
 
 const babelIstanbulPlugin = require.resolve('babel-plugin-istanbul');
 
-const createTransformer = options => {
+const createTransformer = (options = {}) => {
   options = _objectSpread({}, options, {
+    // @ts-ignore: https://github.com/DefinitelyTyped/DefinitelyTyped/pull/32955
     caller: {
       name: 'babel-jest',
       supportsStaticESM: false
@@ -112,9 +113,9 @@ const createTransformer = options => {
     plugins: (options && options.plugins) || [],
     presets: ((options && options.presets) || []).concat(jestPresetPath),
     sourceMaps: 'both'
-  });
+  }); // @ts-ignore: seems like this is removed. Is that true?
+
   delete options.cacheDirectory;
-  delete options.filename;
 
   function loadBabelConfig(cwd, filename) {
     // `cwd` first to allow incoming options to override it
@@ -188,7 +189,7 @@ const createTransformer = options => {
       if (transformOptions && transformOptions.instrument) {
         babelOptions.auxiliaryCommentBefore = ' istanbul ignore next '; // Copied from jest-runtime transform.js
 
-        babelOptions.plugins = babelOptions.plugins.concat([
+        babelOptions.plugins = (babelOptions.plugins || []).concat([
           [
             babelIstanbulPlugin,
             {
@@ -201,10 +202,20 @@ const createTransformer = options => {
       }
 
       const transformResult = (0, _core().transformSync)(src, babelOptions);
-      return transformResult || src;
+
+      if (transformResult && typeof transformResult.code === 'string') {
+        // @ts-ignore: why doesn't TS understand this?
+        return transformResult;
+      }
+
+      return src;
     }
   };
 };
 
-module.exports = createTransformer();
-module.exports.createTransformer = createTransformer;
+const transformer = createTransformer(); // FIXME: This is not part of the exported TS types. When fixed, remember to
+// move @types/babel__core to `dependencies` instead of `devDependencies`
+// (one fix is to use ESM, maybe for Jest 25?)
+
+transformer.createTransformer = createTransformer;
+module.exports = transformer;

Test plan

Green CI

[SimenB]

overwritten on line 47 anyways

[SimenB]

this type is not use now - it will be by jest-runtime later, though

[github-actions]

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