We throw an exception if you tap the screen during load

Inside runApp, we were building the render tree for the app, but we weren't
calling layout, which meant we were running a hit test on a dirty tree. Now
hitTest specifically asserts that the tree is clean and we synchronously call
layout for your app during runApp.

Fixes #1960

packages/flutter/lib/src/rendering/binding.dart

@@ -144,7 +144,7 @@ class FlutterBinding extends HitTestTarget {
       _renderView = renderViewOverride;
     }
     assert(_renderView != null);
-    scheduler.addPersistentFrameCallback(beginFrame);
+    scheduler.addPersistentFrameCallback(_handlePersistentFrameCallback);
 
     assert(_instance == this);
   }
@@ -172,8 +172,12 @@ class FlutterBinding extends HitTestTarget {
       listener(size);
   }
 
+  void _handlePersistentFrameCallback(Duration timeStamp) {
+    beginFrame();
+  }
+
   /// Pump the rendering pipeline to generate a frame for the given time stamp
-  void beginFrame(Duration timeStamp) {
+  void beginFrame() {
     RenderObject.flushLayout();
     _renderView.updateCompositingBits();
     RenderObject.flushPaint();

packages/flutter/lib/src/rendering/box.dart

@@ -543,6 +543,7 @@ abstract class RenderBox extends RenderObject {
   /// coordinate space of the callee.  The callee is responsible for checking
   /// whether the given position is within its bounds.
   bool hitTest(HitTestResult result, { Point position }) {
+    assert(!needsLayout);
     if (position.x >= 0.0 && position.x < _size.width &&
         position.y >= 0.0 && position.y < _size.height) {
       if (hitTestChildren(result, position: position) || hitTestSelf(position)) {

packages/flutter/lib/src/widgets/binding.dart

@@ -22,9 +22,9 @@ class WidgetFlutterBinding extends FlutterBinding {
 
   static WidgetFlutterBinding get instance => FlutterBinding.instance;
 
-  void beginFrame(Duration timeStamp) {
+  void beginFrame() {
     buildDirtyElements();
-    super.beginFrame(timeStamp);
+    super.beginFrame();
     Element.finalizeTree();
   }
 
@@ -74,6 +74,7 @@ class WidgetFlutterBinding extends FlutterBinding {
       container: renderView,
       child: app
     ).attachToRenderTree(_renderViewElement);
+    beginFrame();
   }
 }