Replace stage & es2017 with specific plugins

Also sort out regenerator runtime as it turns out importing
babel-polyfill in your code is insufficient if using webpack
because it's imported too late, so use both that and
regenerator-runtime. Sigh.

.babelrc

@@ -1,3 +1,4 @@
 {
-  "presets": ["react", "es2015", "es2016", "es2017", "stage-2"]
+  "presets": ["react", "es2015", "es2016"],
+  "plugins": ["transform-class-properties", "transform-object-rest-spread", "transform-async-to-generator", "transform-runtime"]
 }

package.json

@@ -66,8 +66,10 @@
     "babel-core": "^6.14.0",
     "babel-eslint": "^6.1.0",
     "babel-loader": "^6.2.5",
+    "babel-plugin-transform-async-to-generator": "^6.16.0",
+    "babel-plugin-transform-class-properties": "^6.16.0",
+    "babel-plugin-transform-object-rest-spread": "^6.16.0",
     "babel-plugin-transform-runtime": "^6.15.0",
-    "babel-polyfill": "^6.5.0",
     "babel-preset-es2015": "^6.16.0",
     "babel-preset-es2016": "^6.16.0",
     "babel-preset-es2017": "^6.16.0",

src/vector/index.js

@@ -18,10 +18,13 @@ limitations under the License.
 
 // for ES6 stuff like startsWith() that Safari doesn't handle
 // and babel doesn't do by default
-// Note we use this, not the babel transform-runtime plugin
+// Note we use this, as well as the babel transform-runtime plugin
 // since transform-runtime does not cover instance methods
 // such as "foobar".includes("foo") which bits of our library
-// code use.
+// code use, but the babel transform-runtime plugin allows the
+// regenerator runtime to be injected early enough in the process
+// (it can't be here as it's too late: the alternative is to put
+// the babel-polyfill as the first 'entry' in the webpack config).
 // https://babeljs.io/docs/plugins/transform-runtime/
 require('babel-polyfill');