chore(pivot): Use Array.flat() instead of flatten from utils

Author: dkamburov

projects/igniteui-angular/src/lib/grids/pivot-grid/pivot-grid.component.ts

@@ -1043,7 +1043,7 @@ export class IgxPivotGridComponent extends IgxGridBaseDirective implements OnIni
         if (this.getDimensionType(dimension) === PivotDimensionType.Row) {
             const relatedDims = PivotUtil.flatten([dimension]).map(x => x.memberName);
             const content = this.rowDimensionContentCollection.filter(x => relatedDims.indexOf(x.dimension.memberName) !== -1);
-            const headers = flatten(content.map(x => x.headerGroups.toArray())).map(x => x.header.refInstance);
+            const headers = content.map(x => x.headerGroups.toArray()).flat().map(x => x.header && x.header.refInstance);
             const autoWidth = this.getLargesContentWidth(headers);
             dimension.width = autoWidth;
             this.pipeTrigger++;

tsconfig.json

@@ -15,7 +15,7 @@
             "node_modules/@types"
         ],
         "lib": [
-            "es2017",
+            "es2019",
             "dom"
         ],
         "paths": {