tabs back btn updates

ionic/components/tabs/tabs.js

@@ -55,15 +55,31 @@ export class Tabs extends ViewController {
     injector: Injector
   ) {
     super(parentViewCtrl, compiler, elementRef, loader, injector);
-    this.item = viewItem;
 
-    this.item.navbarView = () => {
-      let activeTab = this.getActive();
-      if (activeTab && activeTab.instance) {
-        return activeTab.instance.navbarView();
-      }
-      return {};
-    };
+    // Tabs may also be an actual ViewItem which was navigated to
+    // if Tabs is static and not navigated to within a ViewController
+    // then skip this and don't treat it as it's own ViewItem
+    if (viewItem) {
+      this.item = viewItem;
+
+      // special overrides for the Tabs ViewItem
+      // the Tabs ViewItem does not have it's own navbar
+      // so find the navbar it should use within it's active Tab
+      viewItem.navbarView = () => {
+        let activeTab = this.getActive();
+        if (activeTab && activeTab.instance) {
+          return activeTab.instance.navbarView();
+        }
+        return {};
+      };
+
+      // a Tabs ViewItem should not have a back button
+      // enableBack back button should be determined by the
+      // the active ViewItem with a navbar
+      viewItem.enableBack = () => {
+        return false;
+      };
+    }
 
     this.childNavbar(true);
 

ionic/components/view/view-controller.js

@@ -276,11 +276,9 @@ export class ViewController {
   }
 
   swipeBackEnabled() {
-    if (this.items.length > 1) {
-      let activeItem = this.getActive();
-      if (activeItem) {
-        return activeItem.enableBack;
-      }
+    let activeItem = this.getActive();
+    if (activeItem) {
+      return activeItem.enableBack();
     }
     return false;
   }

ionic/components/view/view-item.js

@@ -17,9 +17,6 @@ export class ViewItem {
     this.state = 0;
     this.disposals = [];
 
-    // if it's possible to go back from this nav item
-    this.enableBack = false;
-
     this.protos = {};
     this._nbItms = [];
     this._promises = [];
@@ -32,9 +29,6 @@ export class ViewItem {
   stage(callback) {
     let viewCtrl = this.viewCtrl;
 
-    // update if it's possible to go back from this nav item
-    this.enableBack = viewCtrl && !!viewCtrl.getPrevious(this);
-
     if (this.instance || !viewCtrl) {
       // already compiled this view
       return callback();
@@ -152,6 +146,14 @@ export class ViewItem {
     };
   }
 
+  enableBack() {
+    // update if it's possible to go back from this nav item
+    if (this.viewCtrl) {
+      return !!this.viewCtrl.getPrevious(this);
+    }
+    return false;
+  }
+
   setInstance(instance) {
     this.instance = instance;
   }

ionic/transitions/transition.js

@@ -26,7 +26,7 @@ export class Transition extends Animation {
       let enteringNavbar = this.enteringNavbar = new Animation(enteringItem.navbarElement());
       enteringNavbar.before.addClass(SHOW_NAVBAR_CSS);
 
-      if (enteringItem.enableBack) {
+      if (enteringItem.enableBack()) {
         // only animate in the back button if the entering view has it enabled
         let enteringBackButton = this.enteringBackButton = new Animation(enteringItem.backButtonElement());
         enteringBackButton