chore(table): add sort accessor type to data source (#8222)

src/lib/table/table-data-source.ts

@@ -95,7 +95,8 @@ export class MatTableDataSource<T> implements DataSource<T> {
    * @param data Data object that is being accessed.
    * @param sortHeaderId The name of the column that represents the data.
    */
-  sortingDataAccessor = (data: T, sortHeaderId: string): string|number => {
+  sortingDataAccessor: ((data: T, sortHeaderId: string) => string|number) =
+      (data: T, sortHeaderId: string): string|number => {
     const value: any = data[sortHeaderId];
 
     // If the value is a string and only whitespace, return the value.