Add assert to verify that layout correctly set a box

@@ -50,6 +50,7 @@ class LimitedBox extends SingleChildWidget {
     if (child != null) {
       child.layout(context, _limitConstraints(constraints),
           parentUsesSize: true);
+      assert(child.box != null);
       size = constraints.constrain(child.box.size);
     } else {
       size = _limitConstraints(constraints).constrain(PdfPoint.zero);
@@ -73,6 +74,7 @@ class Padding extends SingleChildWidget {
     if (child != null) {
       final BoxConstraints childConstraints = constraints.deflate(padding);
       child.layout(context, childConstraints, parentUsesSize: parentUsesSize);
+      assert(child.box != null);
       box = constraints.constrainRect(
           width: child.box.width + padding.horizontal,
           height: child.box.height + padding.vertical);
@@ -239,6 +241,7 @@ class Align extends SingleChildWidget {
 
     if (child != null) {
       child.layout(context, constraints.loosen(), parentUsesSize: true);
+      assert(child.box != null);
 
       box = constraints.constrainRect(
           width: shrinkWrapWidth
@@ -272,6 +275,7 @@ class ConstrainedBox extends SingleChildWidget {
     if (child != null) {
       child.layout(context, this.constraints.enforce(constraints),
           parentUsesSize: true);
+      assert(child.box != null);
       box = child.box;
     } else {
       box = PdfRect.fromPoints(PdfPoint.zero,
@@ -308,6 +312,7 @@ class FittedBox extends SingleChildWidget {
     PdfPoint size;
     if (child != null) {
       child.layout(context, const BoxConstraints(), parentUsesSize: true);
+      assert(child.box != null);
       size = constraints
           .constrainSizeAndAttemptToPreserveAspectRatio(child.box.size);
     } else {
@@ -397,6 +402,7 @@ class AspectRatio extends SingleChildWidget {
     if (child != null)
       child.layout(context,
           BoxConstraints.tightFor(width: box.width, height: box.height));
+    assert(child.box != null);
   }
 }
 
@@ -420,6 +426,7 @@ class CustomPaint extends SingleChildWidget {
     if (child != null) {
       child.layout(context, constraints.tighten(width: size.x, height: size.y),
           parentUsesSize: parentUsesSize);
+      assert(child.box != null);
       box = child.box;
     } else {
       box = PdfRect.fromPoints(PdfPoint.zero, constraints.constrain(size));

@@ -152,6 +152,7 @@ class Page {
     if (child != null) {
       final EdgeInsets _margin = margin;
       child.layout(context, constraints, parentUsesSize: parentUsesSize);
+      assert(child.box != null);
       child.box = PdfRect(
           _margin.left,
           pageFormat.height - child.box.height - _margin.top,

@@ -260,6 +260,7 @@ class Flex extends MultiChildWidget {
           }
         }
         child.layout(context, innerConstraints, parentUsesSize: true);
+        assert(child.box != null);
         allocatedSize += _getMainSize(child);
         crossSize = math.max(crossSize, _getCrossSize(child));
       }
@@ -329,6 +330,7 @@ class Flex extends MultiChildWidget {
             }
           }
           child.layout(context, innerConstraints, parentUsesSize: true);
+          assert(child.box != null);
           final double childSize = _getMainSize(child);
           assert(childSize <= maxChildExtent);
           allocatedSize += childSize;

@@ -138,6 +138,7 @@ class GridView extends MultiChildWidget implements SpanningWidget {
         math.min(children.length,
             _context.firstChild + crossAxisCount * _mainAxisCount))) {
       child.layout(context, innerConstraints);
+      assert(child.box != null);
 
       switch (direction) {
         case Axis.vertical:

@@ -143,6 +143,7 @@ class MultiPage extends Page {
           final Widget headerWidget = header(context);
           if (headerWidget != null) {
             headerWidget.layout(context, constraints, parentUsesSize: false);
+            assert(headerWidget.box != null);
             _paintChild(context, headerWidget, _margin.left,
                 offsetStart - headerWidget.box.height, pageFormat.height);
             offsetStart -= headerWidget.box.height;
@@ -153,6 +154,7 @@ class MultiPage extends Page {
           final Widget footerWidget = footer(context);
           if (footerWidget != null) {
             footerWidget.layout(context, constraints, parentUsesSize: false);
+            assert(footerWidget.box != null);
             _paintChild(context, footerWidget, _margin.left, _margin.bottom,
                 pageFormat.height);
             offsetEnd += footerWidget.box.height;
@@ -167,6 +169,7 @@ class MultiPage extends Page {
       }
 
       child.layout(context, constraints, parentUsesSize: false);
+      assert(child.box != null);
 
       // What to do if the widget is too big for the page?
       if (offsetStart - child.box.height < offsetEnd) {
@@ -191,6 +194,7 @@ class MultiPage extends Page {
         child.layout(
             context, constraints.copyWith(maxHeight: offsetStart - offsetEnd),
             parentUsesSize: false);
+        assert(child.box != null);
         _paintChild(context, child, _margin.left,
             offsetStart - child.box.height, pageFormat.height);
 

@@ -100,6 +100,7 @@ class Stack extends MultiChildWidget {
         hasNonPositionedChildren = true;
 
         child.layout(context, nonPositionedConstraints, parentUsesSize: true);
+        assert(child.box != null);
 
         final PdfRect childSize = child.box;
         width = math.max(width, childSize.width);
@@ -137,6 +138,7 @@ class Stack extends MultiChildWidget {
               childConstraints.tighten(height: positioned.height);
 
         positioned.layout(context, childConstraints, parentUsesSize: true);
+        assert(positioned.box != null);
 
         double x;
         if (positioned.left != null) {

@@ -163,6 +163,7 @@ class Table extends Widget implements SpanningWidget {
       int n = 0;
       for (Widget child in row.children) {
         child.layout(context, const BoxConstraints());
+        assert(child.box != null);
         final double calculatedWidth =
             child.box.width == double.infinity ? 0.0 : child.box.width;
         final double childFlex = child is Expanded
@@ -227,6 +228,7 @@ class Table extends Widget implements SpanningWidget {
         final BoxConstraints childConstraints =
             BoxConstraints.tightFor(width: _widths[n]);
         child.layout(context, childConstraints);
+        assert(child.box != null);
         child.box = PdfRect(x, totalHeight, child.box.width, child.box.height);
         x += _widths[n];
         lineHeight = math.max(lineHeight, child.box.height);

@@ -104,6 +104,7 @@ abstract class StatelessWidget extends Widget {
 
     if (_child != null) {
       _child.layout(context, constraints, parentUsesSize: parentUsesSize);
+      assert(_child.box != null);
       box = _child.box;
     } else {
       box = PdfRect.fromPoints(PdfPoint.zero, constraints.smallest);
@@ -139,6 +140,7 @@ abstract class SingleChildWidget extends Widget {
       {bool parentUsesSize = false}) {
     if (child != null) {
       child.layout(context, constraints, parentUsesSize: parentUsesSize);
+      assert(child.box != null);
       box = child.box;
     } else {
       box = PdfRect.fromPoints(PdfPoint.zero, constraints.smallest);