better feedback for null child to a Block or Column

Fixes #1687.

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

@@ -272,8 +272,10 @@ abstract class OneChildRenderObjectWidget extends RenderObjectWidget {
 /// storage for that child list, it doesn't actually provide the updating
 /// logic.)
 abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
-  const MultiChildRenderObjectWidget({ Key key, this.children })
-    : super(key: key);
+  MultiChildRenderObjectWidget({ Key key, this.children })
+    : super(key: key) {
+    assert(!children.any((Widget child) => child == null));
+  }
 
   final List<Widget> children;
 

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

@@ -384,7 +384,9 @@ class Block extends StatelessComponent {
     this.initialScrollOffset,
     this.scrollDirection: ScrollDirection.vertical,
     this.onScroll
-  }) : super(key: key);
+  }) : super(key: key) {
+    assert(!children.any((Widget child) => child == null));
+  }
 
   final List<Widget> children;
   final double initialScrollOffset;