feat(flow): fix webpack bundling when Flow annotations are used

js-utils has some, so we need the preset or webpack fails with a syntax error.

package-lock.json

       "integrity": "sha1-cKFITw+QiaToStRLrDU8lbmxJyE=",
       "dev": true
     },
+    "babel-plugin-syntax-flow": {
+      "version": "6.18.0",
+      "resolved": "http://registry.npmjs.org/babel-plugin-syntax-flow/-/babel-plugin-syntax-flow-6.18.0.tgz",
+      "integrity": "sha1-TDqyCiryaqIM0lmVw5jE63AxDI0=",
+      "dev": true
+    },
     "babel-plugin-syntax-object-rest-spread": {
       "version": "6.13.0",
       "resolved": "https://registry.npmjs.org/babel-plugin-syntax-object-rest-spread/-/babel-plugin-syntax-object-rest-spread-6.13.0.tgz",
         "babel-runtime": "^6.22.0"
       }
     },
+    "babel-plugin-transform-flow-strip-types": {
+      "version": "6.22.0",
+      "resolved": "https://registry.npmjs.org/babel-plugin-transform-flow-strip-types/-/babel-plugin-transform-flow-strip-types-6.22.0.tgz",
+      "integrity": "sha1-hMtnKTXUNxT9wyvOhFaNh0Qc988=",
+      "dev": true,
+      "requires": {
+        "babel-plugin-syntax-flow": "^6.18.0",
+        "babel-runtime": "^6.22.0"
+      }
+    },
     "babel-plugin-transform-object-rest-spread": {
       "version": "6.26.0",
       "resolved": "https://registry.npmjs.org/babel-plugin-transform-object-rest-spread/-/babel-plugin-transform-object-rest-spread-6.26.0.tgz",
         "semver": "^5.3.0"
       }
     },
+    "babel-preset-flow": {
+      "version": "6.23.0",
+      "resolved": "https://registry.npmjs.org/babel-preset-flow/-/babel-preset-flow-6.23.0.tgz",
+      "integrity": "sha1-5xIYiHCFrpoktb5Baa/7WZgWxJ0=",
+      "dev": true,
+      "requires": {
+        "babel-plugin-transform-flow-strip-types": "^6.22.0"
+      }
+    },
     "babel-preset-stage-1": {
       "version": "6.24.1",
       "resolved": "https://registry.npmjs.org/babel-preset-stage-1/-/babel-preset-stage-1-6.24.1.tgz",

package.json

     "babel-eslint": "8.0.3",
     "babel-loader": "7.1.2",
     "babel-preset-env": "1.6.1",
+    "babel-preset-flow": "6.23.0",
     "babel-preset-stage-1": "6.24.1",
     "core-js": "2.5.1",
     "eslint": "4.12.1",

webpack.config.js

                         // so that webpack may do tree shaking.
                         { modules: false }
                     ],
+                    'flow',
                     'stage-1'
                 ]
             },