throw error for view controller push

Closes #138

ionic/components/nav/nav.ts

@@ -44,6 +44,9 @@ export class Nav extends ViewController {
     super.onInit();
 
     if (this.root) {
+      if (typeof this.root !== 'function') {
+        throw 'The [root] property in <ion-nav> must be given a reference to a component class from within the constructor.';
+      }
       this.push(this.root);
     }
 

ionic/components/view/view-controller.ts

@@ -64,6 +64,9 @@ export class ViewController extends Ion {
     if (!componentType || !this.app.isEnabled()) {
       return Promise.reject();
     }
+    if (typeof componentType !== 'function') {
+      throw 'Loading component must be a component class, not "' + componentType.toString() + '"';
+    }
 
     let resolve;
     let promise = new Promise(res => { resolve = res; });