fix(select,autocomplete): unable to set custom id on mat-option

Fixes consumers not being allowed to set their own id on a `mat-option`.

Fixes angular#11572.

src/lib/core/option/option.spec.ts

@@ -9,12 +9,12 @@ describe('MatOption component', () => {
   beforeEach(async(() => {
     TestBed.configureTestingModule({
       imports: [MatOptionModule],
-      declarations: [OptionWithDisable]
+      declarations: [BasicOption]
     }).compileComponents();
   }));
 
   it('should complete the `stateChanges` stream on destroy', () => {
-    const fixture = TestBed.createComponent(OptionWithDisable);
+    const fixture = TestBed.createComponent(BasicOption);
     fixture.detectChanges();
 
     const optionInstance: MatOption =
@@ -27,14 +27,25 @@ describe('MatOption component', () => {
     subscription.unsubscribe();
   });
 
+  it('should be able to set a custom id', () => {
+    const fixture = TestBed.createComponent(BasicOption);
+
+    fixture.componentInstance.id = 'custom-option';
+    fixture.detectChanges();
+
+    const optionInstance = fixture.debugElement.query(By.directive(MatOption)).componentInstance;
+
+    expect(optionInstance.id).toBe('custom-option');
+  });
+
   describe('ripples', () => {
-    let fixture: ComponentFixture<OptionWithDisable>;
+    let fixture: ComponentFixture<BasicOption>;
     let optionDebugElement: DebugElement;
     let optionNativeElement: HTMLElement;
     let optionInstance: MatOption;
 
     beforeEach(() => {
-      fixture = TestBed.createComponent(OptionWithDisable);
+      fixture = TestBed.createComponent(BasicOption);
       fixture.detectChanges();
 
       optionDebugElement = fixture.debugElement.query(By.directive(MatOption));
@@ -73,8 +84,9 @@ describe('MatOption component', () => {
 });
 
 @Component({
-  template: `<mat-option [disabled]="disabled"></mat-option>`
+  template: `<mat-option [id]="id" [disabled]="disabled"></mat-option>`
 })
-class OptionWithDisable {
+class BasicOption {
   disabled: boolean;
+  id: string;
 }

src/lib/core/option/option.ts

@@ -88,21 +88,20 @@ export class MatOption implements AfterViewChecked, OnDestroy {
   private _selected = false;
   private _active = false;
   private _disabled = false;
-  private _id = `mat-option-${_uniqueIdCounter++}`;
   private _mostRecentViewValue = '';
 
   /** Whether the wrapping component is in multiple selection mode. */
   get multiple() { return this._parent && this._parent.multiple; }
 
-  /** The unique ID of the option. */
-  get id(): string { return this._id; }
-
   /** Whether or not the option is currently selected. */
   get selected(): boolean { return this._selected; }
 
   /** The form value of the option. */
   @Input() value: any;
 
+  /** The unique ID of the option. */
+  @Input() id: string = `mat-option-${_uniqueIdCounter++}`;
+
   /** Whether the option is disabled. */
   @Input()
   get disabled() { return (this.group && this.group.disabled) || this._disabled; }