Update flow 0.34 -> 0.67

This shook out a couple of new warnings, which I also addressed in this
commit.

examples/src/examples.js

@@ -5,7 +5,12 @@ import { StyleSheet } from '../../src/index.js';
 
 import StyleTester from './StyleTester.js';
 
+const root = document.getElementById('root');
+
 StyleSheet.rehydrate(window.renderedClassNames);
-ReactDOM.render(
-    <StyleTester />,
-    document.getElementById('root'));
+if (root) {
+    ReactDOM.render(
+        <StyleTester />,
+        root
+    );
+}

package.json

@@ -54,7 +54,7 @@
     "eslint": "^3.7.1",
     "eslint-config-standard-react": "^4.2.0",
     "eslint-plugin-react": "^6.3.0",
-    "flow-bin": "^0.34.0",
+    "flow-bin": "^0.67.0",
     "jsdom": "^11.6.2",
     "mkdirp": "^0.5.1",
     "mocha": "^2.3.3",

src/util.js

@@ -108,7 +108,8 @@ export const stringifyAndImportantifyValue = (
 ) /* : string */ => importantify(stringifyValue(key, prop));
 
 // Turn a string into a hash string of base-36 values (using letters and numbers)
-export const hashString = (string /* : string */) /* string */ => stringHash(string).toString(36);
+// eslint-disable-next-line no-unused-vars
+export const hashString = (string /* : string */, key /* : ?string */) /* string */ => stringHash(string).toString(36);
 
 // Hash a javascript object using JSON.stringify. This is very fast, about 3
 // microseconds on my computer for a sample object: