Fix build for Gatsby (WebPack v1).

WebPack version 1, which Gatsby is using, has the Acorn parser set to
parse ES6 and lower. The latest version of WebPack supports parsing up
to ES2017 and lower (webpack/lib/Parser.js), but Gatsby isn't using it.

.babelrc

@@ -14,7 +14,12 @@
         ]
       }
     }],
-    "react"
+    "react",
+    /**
+     * Support WebPack version 1.
+     */
+    "es2016",
+    "es2017",
   ],
   "plugins": [
     "lodash",

package.json

@@ -85,6 +85,8 @@
     "babel-plugin-transform-object-rest-spread": "^6.26.0",
     "babel-plugin-transform-runtime": "^6.23.0",
     "babel-preset-env": "^1.6.0",
+    "babel-preset-es2016": "^6.24.1",
+    "babel-preset-es2017": "^6.24.1",
     "babel-preset-react": "^6.23.0",
     "babel-runtime": "^6.23.0",
     "cross-env": "^5.0.2",

yarn.lock

@@ -638,7 +638,7 @@ babel-plugin-syntax-trailing-function-commas@^6.22.0:
   version "6.22.0"
   resolved "https://registry.yarnpkg.com/babel-plugin-syntax-trailing-function-commas/-/babel-plugin-syntax-trailing-function-commas-6.22.0.tgz#ba0360937f8d06e40180a43fe0d5616fff532cf3"
 
-babel-plugin-transform-async-to-generator@^6.22.0:
+babel-plugin-transform-async-to-generator@^6.22.0, babel-plugin-transform-async-to-generator@^6.24.1:
   version "6.24.1"
   resolved "https://registry.yarnpkg.com/babel-plugin-transform-async-to-generator/-/babel-plugin-transform-async-to-generator-6.24.1.tgz#6536e378aff6cb1d5517ac0e40eb3e9fc8d08761"
   dependencies:
@@ -830,7 +830,7 @@ babel-plugin-transform-es2015-unicode-regex@^6.22.0:
     babel-runtime "^6.22.0"
     regexpu-core "^2.0.0"
 
-babel-plugin-transform-exponentiation-operator@^6.22.0:
+babel-plugin-transform-exponentiation-operator@^6.22.0, babel-plugin-transform-exponentiation-operator@^6.24.1:
   version "6.24.1"
   resolved "https://registry.yarnpkg.com/babel-plugin-transform-exponentiation-operator/-/babel-plugin-transform-exponentiation-operator-6.24.1.tgz#2ab0c9c7f3098fa48907772bb813fe41e8de3a0e"
   dependencies:
@@ -949,6 +949,19 @@ babel-preset-env@^1.6.0:
     invariant "^2.2.2"
     semver "^5.3.0"
 
+babel-preset-es2016@^6.24.1:
+  version "6.24.1"
+  resolved "https://registry.yarnpkg.com/babel-preset-es2016/-/babel-preset-es2016-6.24.1.tgz#f900bf93e2ebc0d276df9b8ab59724ebfd959f8b"
+  dependencies:
+    babel-plugin-transform-exponentiation-operator "^6.24.1"
+
+babel-preset-es2017@^6.24.1:
+  version "6.24.1"
+  resolved "https://registry.yarnpkg.com/babel-preset-es2017/-/babel-preset-es2017-6.24.1.tgz#597beadfb9f7f208bcfd8a12e9b2b29b8b2f14d1"
+  dependencies:
+    babel-plugin-syntax-trailing-function-commas "^6.22.0"
+    babel-plugin-transform-async-to-generator "^6.24.1"
+
 babel-preset-flow@^6.23.0:
   version "6.23.0"
   resolved "https://registry.yarnpkg.com/babel-preset-flow/-/babel-preset-flow-6.23.0.tgz#e71218887085ae9a24b5be4169affb599816c49d"