Prevent Babel registration from transforming plugins with arbitrary c…

…onfig (#1082)

Summary:
X-link: facebook/react-native#39429


Using `require` after `const registerFn = require('babel/register')` but before `registerFn(config)` causes Babel to transform required code with the default configuration (ie, using a nearby `babel.config.js`, if available).

This was causing the Babel plugins loaded by `metro-babel-register` to be (unnecessarily) transformed according to `babel.config.js`, which actually fails if the plugins/presets referenced in `babel.config.js` themselves require transformation.

This ensures no code is loaded in between registering Babel as a side effect of requiring Babel register, and replacing that hook with something explicitly configured.

## React Native
Changelog: [Internal]

## Metro
```
* **[Fix]:** `metro-babel-register` prevent arbitrary transformation of Babel plugins during registration setup
```

Differential Revision: D49238671

packages/metro-babel-register/src/babel-register.js

@@ -22,7 +22,18 @@ const path = require('path');
 let _only /*: $ReadOnlyArray<RegExp | string> */ = [];
 
 function register(onlyList /*: $ReadOnlyArray<RegExp | string> */) {
-  require('@babel/register')({
+  // NB: `require('@babel/register')` registers Babel as a side-effect, and
+  // also returns a register function that overrides the first registration
+  // when called.
+  //
+  // If `require` is used between the two registrations, Babel will behave in
+  // its default mode, searching for a config file and loading whatever
+  // plugins/presets are specified within, to compile whatever is `require`d.
+  //
+  // Since our `config()` uses `require` to load plugins, and we don't want
+  // these plugins to be compiled with an arbitrary Babel config, we must
+  // prepare the config object before calling `require('@babel/register')`.
+  const registerConfig = {
     ...config(onlyList),
     extensions: [
       '.ts',
@@ -34,7 +45,9 @@ function register(onlyList /*: $ReadOnlyArray<RegExp | string> */) {
       '.js',
       '.mjs',
     ],
-  });
+  };
+
+  require('@babel/register')(registerConfig);
 }
 
 function config(