Re-define browser support matrix

Currently supported browsers and engines: Firefox 52+, Chrome 55+, Edge 15+,
Safari 10.1+, iOS Safari 10.3+ and node 8.9+.

babel_config.js

@@ -6,12 +6,28 @@ export default {
       modules: false,
       targets: {
         browsers: [
-          ">1%",
-          "last 4 versions",
-          "Firefox ESR",
-          "not ie < 9"
-        ]
-      }
+          "Firefox >= 52",
+          "FirefoxAndroid >= 52",
+          "Chrome >= 55",
+          "ChromeAndroid >= 55",
+          "Edge >= 15",
+          "Safari >= 10.1",
+          "iOS >= 10.3",
+        ],
+        node: "8.9",
+      },
+      exclude: [
+        // Exclude regeneratorRuntime explicitly because babel-preset-env
+        // incorrectly transpiles async functions for Firefox 52.
+        // See https://github.com/babel/babel/issues/8086.
+        "transform-regenerator"
+      ],
     }]
+  ],
+  plugins: [
+    // Shipped in Firefox 57, Chrome 63
+    "@babel/plugin-proposal-async-generator-functions",
+    // Shipped in Firefox 55, Chrome 60
+    "@babel/plugin-proposal-object-rest-spread"
   ]
 };