extract get view index into data source view method

Reviewed By: ivanmisuno

Differential Revision: D46224361

fbshipit-source-id: 193becb59eee469ba3e68611e588671947406f7e

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

@@ -584,6 +584,10 @@ export class DataSourceView<T, KeyType> {
     }
   }
 
+  getViewIndex(entry: T): number {
+    return this.output(0, Infinity).indexOf(entry);
+  }
+
   public setWindow(start: number, end: number) {
     this.windowStart = start;
     this.windowEnd = end;