fix(dialog): set aria-labelledby based on the md-dialog-title

* [Based on the accessibility guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/#dialog_modal), these changes add the `aria-labelledby` to dialog that use `md-dialog-title`, which causes the screen reader to read out the title. E.g. before NVDA would read out "Dialog", but now it reads out "Neptune dialog".
* Adds the MdDialogContainer instance to the dialog injector.
* Slight refactor of the dialog injector to make a little easier to add more tokens.

Author: crisbeto

src/lib/dialog/dialog-container.ts

@@ -66,6 +66,7 @@ export function throwMdDialogContentAlreadyAttachedError() {
   host: {
     'class': 'mat-dialog-container',
     '[attr.role]': '_config?.role',
+    '[attr.aria-labelledby]': '_ariaLabelledBy',
     '[@slideDialog]': '_state',
     '(@slideDialog.done)': '_onAnimationDone($event)',
   },
@@ -92,6 +93,9 @@ export class MdDialogContainer extends BasePortalHost {
   /** Emits the current animation state whenever it changes. */
   _onAnimationStateChange = new EventEmitter<AnimationEvent>();
 
+  /** ID of the element that should be considered as the dialog's label. */
+  _ariaLabelledBy: string | null = null;
+
   constructor(
     private _ngZone: NgZone,
     private _elementRef: ElementRef,

src/lib/dialog/dialog-content-directives.ts

@@ -6,9 +6,12 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Directive, Input} from '@angular/core';
+import {Directive, Input, Optional, OnInit} from '@angular/core';
 import {MdDialogRef} from './dialog-ref';
+import {MdDialogContainer} from './dialog-container';
 
+/** Counter used to generate unique IDs for dialog elements. */
+let dialogElementUid = 0;
 
 /**
  * Button that will close the current dialog.
@@ -40,9 +43,22 @@ export class MdDialogClose {
  */
 @Directive({
   selector: '[md-dialog-title], [mat-dialog-title], [mdDialogTitle], [matDialogTitle]',
-  host: {'class': 'mat-dialog-title'},
+  host: {
+    'class': 'mat-dialog-title',
+    '[id]': 'id',
+  },
 })
-export class MdDialogTitle { }
+export class MdDialogTitle implements OnInit {
+  @Input() id = `md-dialog-title-${dialogElementUid++}`;
+
+  constructor(@Optional() private _container: MdDialogContainer) { }
+
+  ngOnInit() {
+    if (this._container && !this._container._ariaLabelledBy) {
+      Promise.resolve().then(() => this._container._ariaLabelledBy = this.id);
+    }
+  }
+}
 
 
 /**

src/lib/dialog/dialog-injector.ts

@@ -6,25 +6,21 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Injector, InjectionToken} from '@angular/core';
+import {Injector} from '@angular/core';
 import {MdDialogRef} from './dialog-ref';
-
-export const MD_DIALOG_DATA = new InjectionToken<any>('MdDialogData');
+import {MdDialogContainer} from './dialog-container';
 
 /** Custom injector type specifically for instantiating components with a dialog. */
 export class DialogInjector implements Injector {
   constructor(
     private _parentInjector: Injector,
-    private _dialogRef: MdDialogRef<any>,
-    private _data: any) { }
+    private _customTokens: Map<any, any>) { }
 
   get(token: any, notFoundValue?: any): any {
-    if (token === MdDialogRef) {
-      return this._dialogRef;
-    }
+    const value = this._customTokens.get(token);
 
-    if (token === MD_DIALOG_DATA) {
-      return this._data;
+    if (typeof value !== 'undefined') {
+      return value;
     }
 
     return this._parentInjector.get<any>(token, notFoundValue);

src/lib/dialog/dialog.spec.ts

@@ -21,11 +21,10 @@ import {NoopAnimationsModule} from '@angular/platform-browser/animations';
 import {Location} from '@angular/common';
 import {SpyLocation} from '@angular/common/testing';
 import {MdDialogModule} from './index';
-import {MdDialog} from './dialog';
+import {MdDialog, MD_DIALOG_DATA} from './dialog';
 import {MdDialogContainer} from './dialog-container';
 import {OverlayContainer, ESCAPE} from '../core';
 import {MdDialogRef} from './dialog-ref';
-import {MD_DIALOG_DATA} from './dialog-injector';
 import {dispatchKeyboardEvent} from '../core/testing/dispatch-events';
 
 
@@ -669,6 +668,17 @@ describe('MdDialog', () => {
       });
     }));
 
+    it('should set the aria-labelled by attribute to the id of the title', async(() => {
+      let title = overlayContainerElement.querySelector('[md-dialog-title]');
+      let container = overlayContainerElement.querySelector('md-dialog-container');
+
+      viewContainerFixture.whenStable().then(() => {
+        expect(title.id).toBeTruthy('Expected title element to have an id.');
+        expect(container.getAttribute('aria-labelledby'))
+            .toBe(title.id, 'Expected the aria-labelledby to match the title id.');
+      });
+    }));
+
   });
 });
 

src/lib/dialog/dialog.ts

@@ -6,7 +6,15 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {Injector, ComponentRef, Injectable, Optional, SkipSelf, TemplateRef} from '@angular/core';
+import {
+  Injector,
+  InjectionToken,
+  ComponentRef,
+  Injectable,
+  Optional,
+  SkipSelf,
+  TemplateRef,
+} from '@angular/core';
 import {Location} from '@angular/common';
 import {Observable} from 'rxjs/Observable';
 import {Subject} from 'rxjs/Subject';
@@ -25,6 +33,8 @@ import {MdDialogRef} from './dialog-ref';
 import {MdDialogContainer} from './dialog-container';
 import {TemplatePortal} from '../core/portal/portal';
 
+export const MD_DIALOG_DATA = new InjectionToken<any>('MdDialogData');
+
 
 /**
  * Service to open Material Design modal dialogs.
@@ -187,17 +197,12 @@ export class MdDialog {
       });
     }
 
-    // We create an injector specifically for the component we're instantiating so that it can
-    // inject the MdDialogRef. This allows a component loaded inside of a dialog to close itself
-    // and, optionally, to return a value.
-    let userInjector = config && config.viewContainerRef && config.viewContainerRef.injector;
-    let dialogInjector = new DialogInjector(userInjector || this._injector, dialogRef, config.data);
-
     if (componentOrTemplateRef instanceof TemplateRef) {
       dialogContainer.attachTemplatePortal(new TemplatePortal(componentOrTemplateRef, null));
     } else {
+      let injector = this._createInjector<T>(config, dialogRef, dialogContainer);
       let contentRef = dialogContainer.attachComponentPortal(
-          new ComponentPortal(componentOrTemplateRef, null, dialogInjector));
+          new ComponentPortal(componentOrTemplateRef, null, injector));
       dialogRef.componentInstance = contentRef.instance;
     }
 
@@ -208,6 +213,29 @@ export class MdDialog {
     return dialogRef;
   }
 
+  /**
+   * Creates a custom injector to be used inside the dialog. This allows a component loaded inside
+   * of a dialog to close itself and, optionally, to return a value.
+   * @param config Config object that is used to construct the dialog.
+   * @param dialogRef Reference to the dialog.
+   * @param container Dialog container element that wraps all of the contents.
+   * @returns The custom injector that can be used inside the dialog.
+   */
+  private _createInjector<T>(
+      config: MdDialogConfig,
+      dialogRef: MdDialogRef<T>,
+      dialogContainer: MdDialogContainer): DialogInjector {
+
+    let userInjector = config && config.viewContainerRef && config.viewContainerRef.injector;
+    let injectionTokens = new Map();
+
+    injectionTokens.set(MdDialogRef, dialogRef);
+    injectionTokens.set(MdDialogContainer, dialogContainer);
+    injectionTokens.set(MD_DIALOG_DATA, config.data);
+
+    return new DialogInjector(userInjector || this._injector, injectionTokens);
+  }
+
   /**
    * Removes a dialog from the array of open dialogs.
    * @param dialogRef Dialog to be removed.

src/lib/dialog/index.ts

@@ -59,4 +59,3 @@ export * from './dialog-container';
 export * from './dialog-content-directives';
 export * from './dialog-config';
 export * from './dialog-ref';
-export {MD_DIALOG_DATA} from './dialog-injector';