OnBoardingScreens: Do not let us skip screens

This requires us to dynamically generate the screens based on if we have
completed the screen or not.

lib/screens/onboarding_screens.dart

@@ -16,15 +16,28 @@ class OnBoardingScreen extends StatefulWidget {
 }
 
 class OnBoardingScreenState extends State<OnBoardingScreen> {
+  var _pageInputUrlDone = false;
+  var _pageSshKeyDone = false;
+
   var pageController = PageController();
   final GlobalKey<ScaffoldState> _scaffoldKey = new GlobalKey<ScaffoldState>();
 
   @override
   Widget build(BuildContext context) {
-    var pageView = PageView(
+    var pageCount = 1;
+    if (_pageInputUrlDone) {
+      pageCount++;
+    }
+    if (_pageSshKeyDone) {
+      pageCount++;
+    }
+
+    var pageView = PageView.builder(
       controller: pageController,
-      children: <Widget>[
+      itemBuilder: (BuildContext context, int pos) {
-        OnBoardingGitUrl(doneFunction: (String sshUrl) {
+        if (pos == 0) {
+          return OnBoardingGitUrl(doneFunction: (String sshUrl) {
+            setPageInputUrlDone();
             pageController.nextPage(
               duration: Duration(milliseconds: 200),
               curve: Curves.easeIn,
@@ -33,20 +46,28 @@ class OnBoardingScreenState extends State<OnBoardingScreen> {
             SharedPreferences.getInstance().then((SharedPreferences pref) {
               pref.setString("sshCloneUrl", sshUrl);
             });
-        }),
+          });
-        OnBoardingSshKey(
+        }
+        if (pos == 1) {
+          return OnBoardingSshKey(
             doneFunction: () {
+              setPageSshKeyDone();
               pageController.nextPage(
                 duration: Duration(milliseconds: 200),
                 curve: Curves.easeIn,
               );
             },
             scaffoldKey: _scaffoldKey,
-        ),
+          );
-        OnBoardingGitClone(
+        }
+
+        if (pos == 2) {
+          return OnBoardingGitClone(
             doneFunction: this.widget.onBoardingCompletedFunction,
-        ),
+          );
-      ],
+        }
+      },
+      itemCount: pageCount,
     );
 
     return new Scaffold(
@@ -59,6 +80,18 @@ class OnBoardingScreenState extends State<OnBoardingScreen> {
       ),
     );
   }
+
+  void setPageInputUrlDone() {
+    setState(() {
+      this._pageInputUrlDone = true;
+    });
+  }
+
+  void setPageSshKeyDone() {
+    setState(() {
+      this._pageSshKeyDone = true;
+    });
+  }
 }
 
 class OnBoardingGitUrl extends StatefulWidget {