fix(overlay): onWillDismiss is called as expected

fixes ionic-team#11702

src/navigation/nav-controller-base.ts

@@ -775,7 +775,10 @@ export class NavControllerBase extends Ion implements NavController {
     if (enteringView || leavingView) {
       this._zone.run(() => {
         // Here, the order is important. WillLeave must be called before WillEnter.
-        leavingView && this._willLeave(leavingView, !enteringView);
+        if (leavingView) {
+          const willUnload = enteringView ? leavingView.index > enteringView.index : true;
+          this._willLeave(leavingView, willUnload);
+        }
         enteringView && this._willEnter(enteringView);
       });
     }
@@ -828,6 +831,8 @@ export class NavControllerBase extends Ion implements NavController {
     // ok, cleanup time!! Destroy all of the views that are
     // INACTIVE and come after the active view
     const activeViewIndex = this._views.indexOf(activeView);
+    assert(activeViewIndex >= 0, 'active index is invalid');
+
     const views = this._views;
     let reorderZIndexes = false;
     let view: ViewController;
@@ -1079,7 +1084,8 @@ export class NavControllerBase extends Ion implements NavController {
       view = this.getActive();
     }
     const views = this._views;
-    return views[views.indexOf(view) - 1];
+    const index = views.indexOf(view);
+    return (index >= 0) ? views[index - 1] : null;
   }
 
   first(): ViewController {
@@ -1089,7 +1095,8 @@ export class NavControllerBase extends Ion implements NavController {
 
   last(): ViewController {
     // returns the last page in this nav controller's stack.
-    return this._views[this._views.length - 1];
+    const views = this._views;
+    return views[views.length - 1];
   }
 
   indexOf(view: ViewController): number {
@@ -1101,9 +1108,6 @@ export class NavControllerBase extends Ion implements NavController {
     return this._views.length;
   }
 
-  /**
-   * Return the stack of views in this NavController.
-   */
   getViews(): Array<ViewController> {
     return this._views;
   }

src/navigation/nav-controller.ts

@@ -576,7 +576,6 @@ export abstract class NavController {
    */
   abstract length(): number;
 
-
   /**
    * Returns the current stack of views in this nav controller.
    * @returns {Array<ViewController>} the stack of view controllers in this nav controller.