Adds support for nullish coalescing operator (#2076)

Summary:
**Summary**

Adds support for nullish coalescing operator which was missing from the babel config of the OSS repo.

**Test Plan**

`yarn`
Verify Travis CI tests pass
Pull Request resolved: #2076

Differential Revision: D15781518

Pulled By: claudiopro

fbshipit-source-id: 4d6deb3503dfbbcf142a8dbaa10ebc2ec8e7a1c4

Author: Claudio Procida

.babelrc

@@ -0,0 +1,3 @@
+{
+  "plugins": ["@babel/plugin-proposal-nullish-coalescing-operator"]
+}

gulpfile.js

@@ -48,6 +48,7 @@ var babelOptsJS = {
       rewriteModules: {map: moduleMap},
     }),
   ],
+  plugins: [require('@babel/plugin-proposal-nullish-coalescing-operator')],
 };
 
 var babelOptsFlow = {
@@ -57,6 +58,7 @@ var babelOptsFlow = {
       rewriteModules: {map: moduleMap},
     }),
   ],
+  plugins: [require('@babel/plugin-proposal-nullish-coalescing-operator')],
 };
 
 var COPYRIGHT_HEADER = `/**

meta/bundle-size-stats/Draft.js.json

@@ -42,13 +42,14 @@
   },
   "devDependencies": {
     "@babel/core": "^7.3.4",
+    "@babel/plugin-proposal-nullish-coalescing-operator": "^7.4.4",
     "babel-eslint": "^8.1.1",
     "babel-preset-fbjs": "^3.2.0",
     "cross-env": "^5.1.3",
     "del": "^2.2.0",
     "envify": "^3.4.0",
     "es6-shim": "^0.34.4",
-    "eslint": "^4.2.0",
+    "eslint": "^5.16.0",
     "eslint-config-fbjs": "^2.0.0",
     "eslint-config-prettier": "^2.6.0",
     "eslint-plugin-babel": "^4.1.1",

meta/bundle-size-stats/Draft.min.js.json

@@ -79,6 +79,7 @@ const decorateCharacterList = (
 class ContentBlockNode extends (Record(defaultRecord): any)
   implements BlockNode {
   constructor(props: ContentBlockNodeConfig = defaultRecord) {
+    /* eslint-disable-next-line constructor-super */
     super(decorateCharacterList(props));
   }