Bring back babel for ES5 builds (#3797)

* Bring back babel for ES5 builds

* Remove ts from babel

build-scripts/babel.js

@@ -0,0 +1,36 @@
+module.exports.babelLoaderConfig = ({ latestBuild }) => {
+  if (latestBuild === undefined) {
+    throw Error("latestBuild not defined for babel loader config");
+  }
+  return {
+    test: /\.m?js$/,
+    use: {
+      loader: "babel-loader",
+      options: {
+        presets: [
+          !latestBuild && [
+            require("@babel/preset-env").default,
+            { modules: false },
+          ],
+        ].filter(Boolean),
+        plugins: [
+          // Part of ES2018. Converts {...a, b: 2} to Object.assign({}, a, {b: 2})
+          [
+            "@babel/plugin-proposal-object-rest-spread",
+            { loose: true, useBuiltIns: true },
+          ],
+          // Only support the syntax, Webpack will handle it.
+          "@babel/syntax-dynamic-import",
+          [
+            require("@babel/plugin-proposal-decorators").default,
+            { decoratorsBeforeExport: true },
+          ],
+          [
+            require("@babel/plugin-proposal-class-properties").default,
+            { loose: true },
+          ],
+        ],
+      },
+    },
+  };
+};