feat(dialog): add the ability to close all dialogs

Adds a `closeAll` method that closes all of the currently-open dialogs.

src/lib/dialog/README.md

@@ -7,6 +7,7 @@ MdDialog is a service, which opens dialogs components in the view.
 | Name |  Description |
 | --- | --- |
 | `open(component: ComponentType<T>, config: MdDialogConfig): MdDialogRef<T>` | Creates and opens a dialog matching material spec. |
+| `closeAll(): void` | Closes all of the dialogs that are currently open. |
 
 ### Config
 

src/lib/dialog/dialog.spec.ts

@@ -214,6 +214,18 @@ describe('MdDialog', () => {
     expect(overlayPane.style.right).toBe('125px');
   });
 
+  it('should close all of the dialogs', () => {
+    dialog.open(PizzaMsg);
+    dialog.open(PizzaMsg);
+    dialog.open(PizzaMsg);
+
+    expect(overlayContainerElement.querySelectorAll('md-dialog-container').length).toBe(3);
+
+    dialog.closeAll();
+
+    expect(overlayContainerElement.querySelectorAll('md-dialog-container').length).toBe(0);
+  });
+
   describe('disableClose option', () => {
     it('should prevent closing via clicks on the backdrop', () => {
       dialog.open(PizzaMsg, {

src/lib/dialog/dialog.ts

@@ -20,7 +20,6 @@ export {MdDialogRef} from './dialog-ref';
 
 
 // TODO(jelbourn): add support for opening with a TemplateRef
-// TODO(jelbourn): add `closeAll` method
 // TODO(jelbourn): dialog content directives (e.g., md-dialog-header)
 // TODO(jelbourn): animations
 
@@ -31,6 +30,9 @@ export {MdDialogRef} from './dialog-ref';
  */
 @Injectable()
 export class MdDialog {
+  /** Keeps track of the currently-open dialogs. */
+  private _openDialogs: MdDialogRef<any>[] = [];
+
   constructor(private _overlay: Overlay, private _injector: Injector) { }
 
   /**
@@ -43,8 +45,32 @@ export class MdDialog {
 
     let overlayRef = this._createOverlay(config);
     let dialogContainer = this._attachDialogContainer(overlayRef, config);
+    let dialogRef = this._attachDialogContent(component, dialogContainer, overlayRef);
+
+    this._openDialogs.push(dialogRef);
+
+    dialogRef.afterClosed().subscribe(() => {
+      let index = this._openDialogs.indexOf(dialogRef);
 
-    return this._attachDialogContent(component, dialogContainer, overlayRef);
+      if (index > -1) {
+        this._openDialogs.splice(index, 1);
+      }
+    });
+
+    return dialogRef;
+  }
+
+  /**
+   * Closes all of the currently-open dialogs.
+   */
+  closeAll(): void {
+    // We need to use the reverse `while`, because the array is being modified
+    // as we're going through it.
+    let i = this._openDialogs.length;
+
+    while (i--) {
+      this._openDialogs[i].close();
+    }
   }
 
   /**