polyfill flat() for jest on Node 10.x

packages/builder/tests/fetchBindableProperties.spec.js

@@ -1,4 +1,11 @@
 import fetchBindableProperties from "../src/builderStore/fetchBindableProperties"
+import { flat } from "lodash"
+
+// supports running jest on Node 10.x
+if (!Array.flat) {
+  Array.flat = flat
+}
+
 describe("fetch bindable properties", () => {
 
   it("should return bindable properties from screen components", () => {