Fix issue #862 (#863)

* Fix issue #862

The following changes were applied:

  - Added a condition where the property "file" is checked. If it
    doesn't exist it is copied from the original source file.

* Fix indentation typo

* Fix index.js with 'yarn lint --fix'

packages/gulp-babel-minify/src/index.js

@@ -63,6 +63,9 @@ function gulpBabelMinify(
     if (success) {
       file.contents = new Buffer(result.code);
       if (file.sourceMap) {
+        if (!result.map.hasOwnProperty("file")) {
+          result.map.file = file.sourceMap.file;
+        }
         applySourceMap(file, result.map);
       }
       return callback(null, file);