fix(bidi): preserve user dir attribute (#13859)

Currently we normalize any values that aren't `ltr` or `rtl` to `ltr` in order to have a predictable value when using the directionality in components, however this ends up overwriting the consumer's value which might not necessarily be used by Angular (e.g. setting it to `auto`). These changes switch to preserving the attribute while normalizing the value for when it gets injected.

Fixes #13855.

src/cdk/bidi/dir.ts

@@ -26,15 +26,19 @@ import {Direction, Directionality} from './directionality';
 @Directive({
   selector: '[dir]',
   providers: [{provide: Directionality, useExisting: Dir}],
-  host: {'[dir]': 'dir'},
+  host: {'[attr.dir]': '_rawDir'},
   exportAs: 'dir',
 })
 export class Dir implements Directionality, AfterContentInit, OnDestroy {
-  _dir: Direction = 'ltr';
+  /** Normalized direction that accounts for invalid/unsupported values. */
+  private _dir: Direction = 'ltr';
 
   /** Whether the `value` has been set to its initial value. */
   private _isInitialized: boolean = false;
 
+  /** Direction as passed in by the consumer. */
+  _rawDir: string;
+
   /** Event emitted when the direction changes. */
   @Output('dirChange') change = new EventEmitter<Direction>();
 
@@ -43,7 +47,10 @@ export class Dir implements Directionality, AfterContentInit, OnDestroy {
   get dir(): Direction { return this._dir; }
   set dir(value: Direction) {
     const old = this._dir;
+
+    this._rawDir = value;
     this._dir = (value === 'ltr' || value === 'rtl') ? value : 'ltr';
+
     if (old !== this._dir && this._isInitialized) {
       this.change.emit(this._dir);
     }

src/cdk/bidi/directionality.spec.ts

@@ -11,7 +11,7 @@ describe('Directionality', () => {
 
     TestBed.configureTestingModule({
       imports: [BidiModule],
-      declarations: [ElementWithDir, InjectsDirectionality],
+      declarations: [ElementWithDir, ElementWithPredefinedAutoDir, InjectsDirectionality],
       providers: [{provide: DIR_DOCUMENT, useFactory: () => fakeDocument}],
     }).compileComponents();
   }));
@@ -123,6 +123,17 @@ describe('Directionality', () => {
       expect(fixture.componentInstance.dir.value).toBe('ltr');
     });
 
+    it('should preserve the consumer-provided `dir` attribute while ' +
+      'normalizing the directive value', () => {
+        const fixture = TestBed.createComponent(ElementWithPredefinedAutoDir);
+        fixture.detectChanges();
+
+        const element = fixture.nativeElement.querySelector('div');
+
+        expect(element.getAttribute('dir')).toBe('auto');
+        expect(fixture.componentInstance.dir.value).toBe('ltr');
+      });
+
   });
 });
 
@@ -140,6 +151,13 @@ class ElementWithDir {
   changeCount = 0;
 }
 
+@Component({
+  template: '<div dir="auto"></div>'
+})
+class ElementWithPredefinedAutoDir {
+  @ViewChild(Dir) dir: Dir;
+}
+
 /** Test component with Dir directive. */
 @Component({
   selector: 'injects-directionality',