Fix minified source maps

Summary: @​public

The source maps generated by uglify are already stringified, and therefore were
being stringified twice.

Reviewed By: @martinbigio

Differential Revision: D2498242

packager/react-packager/src/Server/index.js

@@ -361,9 +361,14 @@ class Server {
           res.end(bundleSource);
           Activity.endEvent(startReqEventId);
         } else if (requestType === 'map') {
-          var sourceMap = JSON.stringify(p.getSourceMap({
+          var sourceMap = p.getSourceMap({
             minify: options.minify,
-          }));
+          });
+
+          if (typeof sourceMap !== 'string') {
+            sourceMap = JSON.stringify(sourceMap);
+          }
+
           res.setHeader('Content-Type', 'application/json');
           res.end(sourceMap);
           Activity.endEvent(startReqEventId);