fix(Page.type): Add assertion to page.type() (#1301)

This patch starts throwing a nice error when `page.type` is called
with non-existent selector.

lib/Page.js

@@ -795,6 +795,7 @@ class Page extends EventEmitter {
    */
   async type(selector, text, options) {
     const handle = await this.$(selector);
+    console.assert(handle, 'No node found for selector: ' + selector);
     await handle.type(text, options);
     await handle.dispose();
   }