Typecheck sources/AddSourceMapURLDialog.js with TypeScript

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Bug: 1011811
Change-Id: I2088e7ba0c91f2cb56a5eee4cb9833119ea0f96f
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2450033
Commit-Queue: Tim van der Lippe <[email protected]>
Auto-Submit: Tim van der Lippe <[email protected]>
Reviewed-by: Paul Lewis <[email protected]>
diff --git a/front_end/ui/Dialog.js b/front_end/ui/Dialog.js
index d39421fd..3cdfa26 100644
--- a/front_end/ui/Dialog.js
+++ b/front_end/ui/Dialog.js
@@ -74,7 +74,7 @@
 
   /**
    * @override
-   * @param {!Document|!Element=} where
+   * @param {(!Document|!Element)=} where
    */
   show(where) {
     const document = /** @type {!Document} */ (