Prevent mangling of new API name

Without the declare, clients that use ClosureCompiler will rename the
method name, while the gr-diff bundle does not, making it not find the
method when calling cross bundle.

Change-Id: I54f6a2f750c042badb1de8109352f9850c0f20d4

polygerrit-ui/app/api/diff.ts

diff --git a/polygerrit-ui/app/api/diff.ts b/polygerrit-ui/app/api/diff.ts
index 724141f..3f51cf0 100644
--- a/polygerrit-ui/app/api/diff.ts
+++ b/polygerrit-ui/app/api/diff.ts
@@ -407,7 +407,7 @@
 }
 
 /** An instance of the GrDiff Webcomponent */
-export interface GrDiff extends HTMLElement {
+export declare interface GrDiff extends HTMLElement {
   /**
    * Return line number element for reading only,
    *