Allow transform of Transformers to return promise

Author: azu

src/TransformOutput.js

@@ -26,17 +26,19 @@ function transform(transformer, transformCode, code) {
         '})',
       ].join('')
     );
-    let result = transformer.transform(
+    let resultPromise = Promise.resolve(transformer.transform(
       realTransformer,
       transformCode,
       code,
-    );
-    let map = null;
-    if (typeof result !== 'string') {
-      map = new SourceMapConsumer(result.map);
-      result = result.code;
-    }
-    return { result, map };
+    ));
+    return resultPromise.then(result => {
+      let map = null;
+      if (typeof result !== 'string') {
+        map = new SourceMapConsumer(result.map);
+        result = result.code;
+      }
+      return { result, map };
+    });
   });
 }