diff -u b/core/package.json b/core/package.json
--- b/core/package.json
+++ b/core/package.json
@@ -10,7 +10,7 @@
   },
   "devDependencies": {
     "babel-core": "6.24.0",
-    "babel-preset-env": "1.2.2",
+    "babel-preset-es2015": "6.16.0",
     "chokidar": "1.6.1",
     "eslint": "3.18.0",
     "eslint-config-airbnb": "14.1.0",
@@ -21,17 +21,7 @@
   },
   "babel": {
     "presets": [
-      [
-        "env",
-        {
-          "targets": {
-            "browsers": [
-              "last 2 versions",
-              "ie >= 9"
-            ]
-          }
-        }
-      ]
+      "es2015"
     ]
   }
 }
reverted:
--- b/core/scripts/js/babel-es6-build.js
+++ a/core/scripts/js/babel-es6-build.js
@@ -33,12 +33,10 @@
     // we've requested for a sourcemap to be written to disk
     let mapLoc = `${fileName}.js.map`;
 
+    fs.writeFile(mapLoc, JSON.stringify(result.map));
+    fs.writeFile(`${fileName}.js`, addSourceMappingUrl(result.code, mapLoc));
+
     log(`'${filePath}' is being processed.`);
-    fs.writeFile(mapLoc, JSON.stringify(result.map), () => {
-      fs.writeFile(`${fileName}.js`, addSourceMappingUrl(result.code, mapLoc), () => {
-        log(`'${filePath}' is finished.`);
-      });
-    });
   });
 };