Add Symbol polyfill to make pretty-format happy

Author: developit

src/jsx.js

@@ -1,3 +1,4 @@
+import './polyfills';
 import renderToString from '.';
 import { indent, encodeEntities, assign } from './util';
 import prettyFormat from 'pretty-format';

src/polyfills.js

@@ -0,0 +1,7 @@
+if (typeof Symbol!=='function') {
+	let c = 0;
+	Symbol = function(s) {		// eslint-disable-line
+		return `@@${s}${++c}`;
+	};
+	Symbol.for = s => `@@${s}`;
+}