Fix codemirror typings for editor test.

packages/codemirror/typings/codemirror/codemirror.d.ts

@@ -59,6 +59,10 @@ declare module 'codemirror' {
       pos: { from: CodeMirror.Position; to: CodeMirror.Position },
       margin?: number
     ): void;
+
+    /** Trigger key events onto the editor instance. Not for production use, only for testing.
+     See this comment: https://github.com/codemirror/CodeMirror/issues/1935#issuecomment-28178991 */
+     triggerOnKeyDown(event: Event): void;
   }
 
   interface Selection {