Fix BABEL_ENV resolution in babel transformer (#446)

Summary:
**Summary**

Fix a regression introduced in `v0.53.0` via c0f6693

We're using some babel plugins in our React Native applications and noticed some weird issues after upgrading to RN 0.60. Upon further investigation, it appeared the issue is coming from metro babel transformer setting `BABEL_ENV` to an incorrect value.

The problem is due to the very unintuitive behavior of assigning values on `process.env` as described in [nodejs doc](https://nodejs.org/dist/latest-v10.x/docs/api/process.html) :
> Assigning a property on process.env will implicitly convert the value to a string. This behavior is deprecated. Future versions of Node.js may throw an error when the value is not a string, number, or boolean.

For example, from a node REPL :
```
> process.env.BABEL_ENV = undefined
undefined
> process.env.BABEL_ENV === undefined
false
> process.env.BABEL_ENV === 'undefined'
true
```

With `BABEL_ENV` not set (undefined), and `options.dev=false` the first run of the `transform` function is properly setting `BABEL_ENV` to `production`. Then, prior to exiting the function, it is [restoring `BABEL_ENV` to its initial value](https://github.com/facebook/metro/blob/v0.56.0/packages/metro-babel-transformer/src/index.js#L74) that was captured upon entering the function. In this case `undefined`, so `BABEL_ENV` is now set to `'undefined'` (string). On subsequent runs of the function it will therefore set `BABEL_ENV` to `'undefined'`.

Given this, the fix is actually straightforward. Only assign `OLD_BABEL_ENV` to `BABEL_ENV` if `OLD_BABEL_ENV` is defined, to avoid running into the problem of having `BABEL_ENV` set to `'undefined'`.
Pull Request resolved: #446

Differential Revision: D17317325

Pulled By: cpojer

fbshipit-source-id: 1b00c126279124470a2de907692363e5aec96ae3

packages/metro-babel-transformer/src/index.js

@@ -71,7 +71,9 @@ function transform({filename, options, plugins, src}: BabelTransformerArgs) {
 
     return {ast, functionMap};
   } finally {
-    process.env.BABEL_ENV = OLD_BABEL_ENV;
+    if (OLD_BABEL_ENV) {
+      process.env.BABEL_ENV = OLD_BABEL_ENV;
+    }
   }
 }
 

packages/metro-react-native-babel-transformer/src/index.js

@@ -172,7 +172,9 @@ function transform({filename, options, src, plugins}: BabelTransformerArgs) {
 
     return {ast: result.ast, functionMap};
   } finally {
-    process.env.BABEL_ENV = OLD_BABEL_ENV;
+    if (OLD_BABEL_ENV) {
+      process.env.BABEL_ENV = OLD_BABEL_ENV;
+    }
   }
 }