use babel-plugin-transform-object-assign

.babelrc

@@ -0,0 +1,22 @@
+{
+  "plugins": ["fast-async",
+              "check-es2015-constants",
+              "transform-es2015-arrow-functions",
+              "transform-es2015-block-scoped-functions",
+              "transform-es2015-block-scoping",
+              "transform-es2015-classes",
+              "transform-es2015-computed-properties",
+              "transform-es2015-destructuring",
+              "transform-es2015-duplicate-keys",
+              "transform-es2015-for-of",
+              "transform-es2015-function-name",
+              "transform-es2015-literals",
+              "transform-es2015-object-super",
+              "transform-es2015-parameters",
+              "transform-es2015-shorthand-properties",
+              "transform-es2015-spread",
+              "transform-es2015-sticky-regex",
+              "transform-es2015-unicode-regex",
+              "transform-object-assign",
+            ]
+}