Bring back required export jsCallDispatcher.findJSFunction (dotnet/extensions#2002)

\n\nCommit migrated from 5350efd2f4

src/JSInterop/Microsoft.JSInterop.JS/src/src/Microsoft.JSInterop.ts

@@ -141,6 +141,14 @@ module DotNet {
    * Receives incoming calls from .NET and dispatches them to JavaScript.
    */
   export const jsCallDispatcher = {
+    /**
+     * Finds the JavaScript function matching the specified identifier.
+     *
+     * @param identifier Identifies the globally-reachable function to be returned.
+     * @returns A Function instance.
+     */
+    findJSFunction, // Note that this is used by the JS interop code inside Mono WebAssembly itself
+
     /**
      * Invokes the specified synchronous JavaScript function.
      *