fix(button-group): Fix a regression that was introduced with the upda…

…te to the new typescript version.

Typescript can now differentiate between properties an get Accessors in the type declarations of the mixins so it broke with the new update

Fixes #1577

libs/barista-components/button-group/src/button-group.ts

@@ -41,12 +41,12 @@ import {
   mixinColor,
   mixinTabIndex,
   _readKeyCode,
+  HasElementRef,
 } from '@dynatrace/barista-components/core';
 
-export class DtButtonGroupBase {
-  disabled: boolean;
-}
-export const _DtButtonGroup = mixinTabIndex(DtButtonGroupBase);
+export const _DtButtonGroup = mixinTabIndex(
+  class {} as Constructor<CanDisable>,
+);
 
 @Component({
   selector: 'dt-button-group',
@@ -156,15 +156,14 @@ export interface DtButtonGroupItemSelectionChange<T> {
 
 export type DtButtonGroupThemePalette = 'main' | 'error';
 export class DtButtonGroupItemBase {
-  /** Whether the button group item is disabled. */
-  disabled: boolean;
   constructor(public _elementRef: ElementRef) {}
 }
+
 export const _DtButtonGroupItem = mixinTabIndex(
-  mixinColor<Constructor<DtButtonGroupItemBase>, DtButtonGroupThemePalette>(
-    DtButtonGroupItemBase,
-    'main',
-  ),
+  mixinColor<
+    Constructor<CanDisable & HasElementRef>,
+    DtButtonGroupThemePalette
+  >(DtButtonGroupItemBase as Constructor<CanDisable & HasElementRef>, 'main'),
 );
 
 @Component({
@@ -247,7 +246,8 @@ export class DtButtonGroupItem<T> extends _DtButtonGroupItem
   constructor(
     private _buttonGroup: DtButtonGroup<T>,
     private _changeDetectorRef: ChangeDetectorRef,
-    _elementRef: ElementRef,
+    /** @internal */
+    public _elementRef: ElementRef,
     private _focusMonitor: FocusMonitor,
   ) {
     super(_elementRef);

libs/barista-components/core/src/common-behaviours/color.ts

@@ -70,7 +70,6 @@ export function mixinColor<
     // tslint:disable-next-line:no-any
     constructor(...args: any[]) {
       super(...args);
-
       // Set the default color that can be specified from the mixin.
       this.color = defaultColor as P;
     }