fix(snack-bar): Clear duration timeout on dismiss

When a duration is passed to MdSnackBar.prototype.openFromComponent(),
a timeout is created. This timeout is now cleared when dismissing the
snackbar.

Fixes #4859

src/lib/snack-bar/snack-bar-ref.ts

@@ -31,6 +31,9 @@ export class MdSnackBarRef<T> {
   /** Subject for notifying the user that the snack bar action was called. */
   private _onAction: Subject<any> = new Subject();
 
+  /** Timeout handle for the dismiss timeout cleanup. */
+  private _dismissTimeoutHandle: number;
+
   constructor(instance: T,
               containerInstance: MdSnackBarContainer,
               private _overlayRef: OverlayRef) {
@@ -47,6 +50,12 @@ export class MdSnackBarRef<T> {
     if (!this._afterClosed.closed) {
       this.containerInstance.exit();
     }
+    clearTimeout(this._dismissTimeoutHandle);
+  }
+
+  /** Dismisses the snack bar after some duration */
+  dismissAfter(duration: number): void {
+    this._dismissTimeoutHandle = setTimeout(() => this.dismiss(), duration);
   }
 
   /** Marks the snackbar action clicked. */

src/lib/snack-bar/snack-bar.ts

@@ -81,7 +81,7 @@ export class MdSnackBar {
     // If a dismiss timeout is provided, set up dismiss based on after the snackbar is opened.
     if (config.duration > 0) {
       snackBarRef.afterOpened().subscribe(() => {
-        setTimeout(() => snackBarRef.dismiss(), config.duration);
+        snackBarRef.dismissAfter(config.duration);
       });
     }