Typecheck JS files for chrome://help before doing import transition

third_party/closure_compiler/externs/pending_compiler_externs.js

Index: third_party/closure_compiler/externs/pending_compiler_externs.js
diff --git a/third_party/closure_compiler/externs/pending_compiler_externs.js b/third_party/closure_compiler/externs/pending_compiler_externs.js
index 9945b2e99ae5273efe7268f83ff55da28088843b..797feee59099116c10630ec9c93779c3686c8e0d 100644
--- a/third_party/closure_compiler/externs/pending_compiler_externs.js
+++ b/third_party/closure_compiler/externs/pending_compiler_externs.js
@@ -13,3 +13,13 @@
  * @return {!HTMLDocument}
  */
 DOMImplementation.prototype.createHTMLDocument = function(opt_title) {};
+
+/**
+ * @see http://www.whatwg.org/specs/web-apps/current-work/#popstateevent
+ * @constructor
+ * @extends {Event}
+ *
+ * @param {string} type
+ * @param {{state: *}=} opt_eventInitDict
+ */
+function PopStateEvent(type, opt_eventInitDict) {}