feat(dialog): add ariaLabelledBy config option

this change adds the ability to set the `aria-labelledby` of a dialog. user can customize the `aria-labelledby` without mat-dialog-title.

no breaking changes.  existing functionality is preserved without setting the ariaLabelledBy config option.

Author: ijz953

src/lib/dialog/dialog-config.ts

@@ -89,6 +89,9 @@ export class MatDialogConfig<D = any> {
   /** ID of the element that describes the dialog. */
   ariaDescribedBy?: string | null = null;
 
+  /** ID of the element that labels the dialog. */
+  ariaLabelledBy?: string | null = null;
+
   /** Aria label to assign to the dialog element */
   ariaLabel?: string | null = null;
 

src/lib/dialog/dialog-container.ts

@@ -87,7 +87,7 @@ export class MatDialogContainer extends BasePortalOutlet {
   _animationStateChanged = new EventEmitter<AnimationEvent>();
 
   /** ID of the element that should be considered as the dialog's label. */
-  _ariaLabelledBy: string | null = null;
+  _ariaLabelledBy: string | null;
 
   /** ID for the container DOM element. */
   _id: string;
@@ -101,6 +101,7 @@ export class MatDialogContainer extends BasePortalOutlet {
     public _config: MatDialogConfig) {
 
     super();
+    this._ariaLabelledBy = _config.ariaLabelledBy || null;
   }
 
   /**

src/lib/dialog/dialog.spec.ts

@@ -1180,6 +1180,51 @@ describe('MatDialog', () => {
     }
   });
 
+  describe('aria-labelledby', () => {
+    it('should be able to set a custom aria-labelledby', () => {
+      dialog.open(PizzaMsg, {
+        ariaLabelledBy: 'Labelled By',
+        viewContainerRef: testViewContainerRef
+      });
+      viewContainerFixture.detectChanges();
+
+      const container = overlayContainerElement.querySelector('mat-dialog-container')!;
+      expect(container.getAttribute('aria-labelledby')).toBe('Labelled By');
+    });
+
+    it('should not set the aria-labelledby automatically if it has an aria-label ' +
+      'and an aria-labelledby', fakeAsync(() => {
+        dialog.open(ContentElementDialog, {
+          ariaLabel: 'Hello there',
+          ariaLabelledBy: 'Labelled By',
+          viewContainerRef: testViewContainerRef
+        });
+        viewContainerFixture.detectChanges();
+        tick();
+        viewContainerFixture.detectChanges();
+
+        const container = overlayContainerElement.querySelector('mat-dialog-container')!;
+        expect(container.hasAttribute('aria-labelledby')).toBe(false);
+    }));
+
+    it('should set the aria-labelledby attribute to the config provided aria-labelledby ' +
+      'instead of the mat-dialog-title id', fakeAsync(() => {
+        dialog.open(ContentElementDialog, {
+          ariaLabelledBy: 'Labelled By',
+          viewContainerRef: testViewContainerRef
+        });
+        viewContainerFixture.detectChanges();
+        flush();
+        let title = overlayContainerElement.querySelector('[mat-dialog-title]')!;
+        let container = overlayContainerElement.querySelector('mat-dialog-container')!;
+        flush();
+        viewContainerFixture.detectChanges();
+
+        expect(title.id).toBeTruthy('Expected title element to have an id.');
+        expect(container.getAttribute('aria-labelledby')).toBe('Labelled By');
+    }));
+  });
+
   describe('aria-label', () => {
     it('should be able to set a custom aria-label', () => {
       dialog.open(PizzaMsg, {

tools/public_api_guard/lib/dialog.d.ts

@@ -57,6 +57,7 @@ export declare class MatDialogClose implements OnInit, OnChanges {
 export declare class MatDialogConfig<D = any> {
     ariaDescribedBy?: string | null;
     ariaLabel?: string | null;
+    ariaLabelledBy?: string | null;
     autoFocus?: boolean;
     backdropClass?: string;
     closeOnNavigation?: boolean;