feat(dialog): allow for single dialog scroll strategy to be overwritten

Currently we have the `MAT_DIALOG_SCROLL_STRATEGY` token which allows consumers to specify the scroll strategy for all dialogs, however there's no way do so for a single dialog. These changes add an extra property to the dialog config that allow for the scroll strategy to be set.

Fixes angular#8706.

src/lib/dialog/dialog-config.ts

@@ -8,6 +8,7 @@
 
 import {ViewContainerRef} from '@angular/core';
 import {Direction} from '@angular/cdk/bidi';
+import {ScrollStrategy} from '@angular/cdk/overlay';
 
 /** Valid ARIA roles for a dialog element. */
 export type DialogRole = 'dialog' | 'alertdialog';
@@ -94,5 +95,8 @@ export class MatDialogConfig<D = any> {
   /** Whether the dialog should focus the first focusable element on open. */
   autoFocus?: boolean = true;
 
+  /** Scroll strategy to be used for the dialog. */
+  scrollStrategy?: ScrollStrategy;
+
   // TODO(jelbourn): add configuration for lifecycle hooks, ARIA labelling.
 }

src/lib/dialog/dialog.spec.ts

@@ -24,7 +24,7 @@ import {Location} from '@angular/common';
 import {SpyLocation} from '@angular/common/testing';
 import {Directionality} from '@angular/cdk/bidi';
 import {MatDialogContainer} from './dialog-container';
-import {OverlayContainer} from '@angular/cdk/overlay';
+import {OverlayContainer, ScrollStrategy} from '@angular/cdk/overlay';
 import {A, ESCAPE} from '@angular/cdk/keycodes';
 import {dispatchKeyboardEvent} from '@angular/cdk/testing';
 import {MAT_DIALOG_DATA, MatDialog, MatDialogModule, MatDialogRef} from './index';
@@ -599,6 +599,17 @@ describe('MatDialog', () => {
     expect(spy).toHaveBeenCalled();
   }));
 
+  it('should be able to attach a custom scroll strategy', fakeAsync(() => {
+    const scrollStrategy: ScrollStrategy = {
+      attach: () => {},
+      enable: jasmine.createSpy('scroll strategy enable spy'),
+      disable: () => {}
+    };
+
+    dialog.open(PizzaMsg, {scrollStrategy});
+    expect(scrollStrategy.enable).toHaveBeenCalled();
+  }));
+
   describe('passing in data', () => {
     it('should be able to pass in data', () => {
       let config = {

src/lib/dialog/dialog.ts

@@ -9,7 +9,6 @@
 import {Directionality} from '@angular/cdk/bidi';
 import {ESCAPE} from '@angular/cdk/keycodes';
 import {
-  BlockScrollStrategy,
   Overlay,
   OverlayConfig,
   OverlayRef,
@@ -47,7 +46,7 @@ export const MAT_DIALOG_SCROLL_STRATEGY =
 
 /** @docs-private */
 export function MAT_DIALOG_SCROLL_STRATEGY_PROVIDER_FACTORY(overlay: Overlay):
-    () => BlockScrollStrategy {
+    () => ScrollStrategy {
   return () => overlay.scrollStrategies.block();
 }
 
@@ -175,7 +174,7 @@ export class MatDialog {
   private _getOverlayConfig(dialogConfig: MatDialogConfig): OverlayConfig {
     const state = new OverlayConfig({
       positionStrategy: this._overlay.position().global(),
-      scrollStrategy: this._scrollStrategy(),
+      scrollStrategy: dialogConfig.scrollStrategy || this._scrollStrategy(),
       panelClass: dialogConfig.panelClass,
       hasBackdrop: dialogConfig.hasBackdrop,
       direction: dialogConfig.direction,