Add transform-react-jsx-source to react-native preset

Summary:
Putting this up as request for comments.

The PR adds [transform-react-jsx-source](https://github.com/babel/babel/tree/master/packages/babel-plugin-transform-react-jsx-source) to the list of plugins that come by default with the `react-native` preset. It will enable the use of a bunch of really cool tooling around JSX, however those are generally useful only in development mode. Is changing `react-native` preset the right thing to do in this case? Is there a way to enable this transform only in DEV? Should I add this somewhere else?
Closes https://github.com/facebook/react-native/pull/6351

Differential Revision: D3302906

Pulled By: frantic

fbshipit-source-id: 012d3a4142168f9f90d30d1686115d4dc3996eb9

transformer.js

@@ -100,15 +100,22 @@ function buildBabelConfig(filename, options) {
 function transform(src, filename, options) {
   options = options || {};
 
-  const babelConfig = buildBabelConfig(filename, options);
+  const OLD_BABEL_ENV = process.env.BABEL_ENV;
-  const result = babel.transform(src, babelConfig);
+  process.env.BABEL_ENV = options.dev ? 'development' : 'production';
 
-  return {
+  try {
-    ast: result.ast,
+    const babelConfig = buildBabelConfig(filename, options);
-    code: result.code,
+    const result = babel.transform(src, babelConfig);
-    map: result.map,
+
-    filename: filename,
+    return {
-  };
+      ast: result.ast,
+      code: result.code,
+      map: result.map,
+      filename: filename,
+    };
+  } finally {
+    process.env.BABEL_ENV = OLD_BABEL_ENV;
+  }
 }
 
 module.exports = function(data, callback) {