Replace object getter with Object.defineProperty in TestRenderer (#…

…10473)

* Add babel transform for object getters

**what is the change?:**
We were not transforming object getters[1], and our new TestRenderer
uses one.[2]

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get
[2]: https://github.com/facebook/react/blob/master/src/renderers/testing/ReactTestRendererFiberEntry.js#L569

**why make this change?:**
Our internal build system was not supporting object getters.

**test plan:**
`yarn build && yarn test`
Also built and opened the 'packaging' fixture.

Honestly I'm not sure what else to test, this seems pretty low risk.

**issue:**
None opened yet

* Replace object getter with `Object.defineProperty` in TestRenderer

**what is the change?:**
Replaces an Object Getter [1] with `Object.defineProperty`.

[1]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get

**why make this change?:**
Our internal build system does not support object getters.

**test plan:**
We should do follow-up work to ensure that this doesn't come up again -
we can't commit code which uses object getters.

**issue:**
No issue opened yet

* Tweak the Object.defineProperty call in test renderer to make flow pass

**what is the change?:**
- switched from `Object.defineProperties` to `Object.defineProperty`
- set some undefined properties to get flow to pass

**why make this change?:**
- Flow doesn't seem to play nicely with `Object.defineProperty/ies`
  calls, specifically when you implement `get` and `set` methods.
  See facebook/flow#1336
- Switched from `properties` to `property` because it seems more
  conventional in our codebase to use `property`, and I'm only setting
  one anyway.

**test plan:**
`flow`

**issue:**
no issue

* More flow type tweaking

**what is the change?:**
Forced the typing of an argument to 'Object.defineProperty' to be
'Object' to avoid an issue with Flow.

**why make this change?:**
To get tests passing and flow passing.

**test plan:**
`flow && yarn test`

**issue:**

src/renderers/testing/ReactTestRendererFiberEntry.js

@@ -565,13 +565,9 @@ var ReactTestRendererFiber = {
     invariant(root != null, 'something went wrong');
     TestRenderer.updateContainer(element, root, null, null);
 
-    return {
-      get root() {
-        if (root === null || root.current.child === null) {
-          throw new Error("Can't access .root on unmounted test renderer");
-        }
-        return wrapFiber(root.current.child);
-      },
+    var entry = {
+      root: undefined, // makes flow happy
+      // we define a 'getter' for 'root' below using 'Object.defineProperty'
       toJSON() {
         if (root == null || root.current == null || container == null) {
           return null;
@@ -611,6 +607,23 @@ var ReactTestRendererFiber = {
         return TestRenderer.getPublicRootInstance(root);
       },
     };
+
+    Object.defineProperty(
+      entry,
+      'root',
+      ({
+        configurable: true,
+        enumerable: true,
+        get: function() {
+          if (root === null || root.current.child === null) {
+            throw new Error("Can't access .root on unmounted test renderer");
+          }
+          return wrapFiber(root.current.child);
+        },
+      }: Object),
+    );
+
+    return entry;
   },
 
   /* eslint-disable camelcase */