message: Small refactor in `ListPicker`
diff: @@ -54,7 +54,7 @@ export class ListPicker<T extends { id: string; }> extends ThemeableElementBase
  }
  private _handleFilterInput(): void {
  - const filterInput = <HTMLInputElement> this._elementById(ID_FILTER);
  + const filterInput = <HTMLInputElement> DomUtils.getShadowId(this, ID_FILTER);
  this.filter = filterInput.value;
  }
  @@ -221,7 +221,7 @@ export class ListPicker<T extends { id: string; }> extends ThemeableElementBase
  }
  } else {
  - const resultsDiv = this._elementById(ID_RESULTS);
  + const resultsDiv = DomUtils.getShadowId(this, ID_RESULTS);
  // Determine the step size.
  let stepSize = 1;
  @@ -271,7 +271,7 @@ export class ListPicker<T extends { id: string; }> extends ThemeableElementBase
  this.filter = "";
  this._render();
  - const filterInput = <HTMLInputElement> this._elementById(ID_FILTER);
  + const filterInput = <HTMLInputElement> DomUtils.getShadowId(this, ID_FILTER);
  filterInput.focus();
  this._scrollToSelected();
  @@ -286,8 +286,4 @@ export class ListPicker<T extends { id: string; }> extends ThemeableElementBase
  });
  }
  }
  -
  - private _elementById(id: string): HTMLElement {
  - return DomUtils.getShadowId(this, id);
  - }
  }
  