avoid creating a copy of an array of entire view to look up index

Summary: What title says

Reviewed By: lblasa

Differential Revision: D46275672

fbshipit-source-id: 65c6566a042b576b83ababdf6470ddf2f626d3e4

desktop/flipper-plugin-core/src/data-source/DataSource.tsx

@@ -585,7 +585,7 @@ export class DataSourceView<T, KeyType> {
   }
 
   getViewIndex(entry: T): number {
-    return this.output(0, Infinity).indexOf(entry);
+    return this._output.findIndex((x) => x.value === entry);
   }
 
   public setWindow(start: number, end: number) {