Add a more meaningful message to the assertion on children property of...

Add a more meaningful message to the assertion on children property of MultiChildRenderObjectWidget and SliverChildListDelegate. (#32487)

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

@@ -1663,7 +1663,15 @@ abstract class MultiChildRenderObjectWidget extends RenderObjectWidget {
   /// objects.
   MultiChildRenderObjectWidget({ Key key, this.children = const <Widget>[] })
     : assert(children != null),
-      assert(!children.any((Widget child) => child == null)), // https://github.com/dart-lang/sdk/issues/29276
+      assert(() {
+        final int index = children.indexOf(null);
+        if (index >= 0) {
+          throw AssertionError(
+            "The widget's children must not contain any null values, but a null value was found at index $index"
+          );
+        }
+        return true;
+      }()), // https://github.com/dart-lang/sdk/issues/29276
       super(key: key);
 
   /// The widgets below this widget in the tree.

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

@@ -670,7 +670,10 @@ class SliverChildListDelegate extends SliverChildDelegate {
       return null;
     Widget child = children[index];
     final Key key = child.key != null? _SaltedValueKey(child.key) : null;
-    assert(child != null);
+    assert(
+      child != null,
+      "The sliver's children must not contain null values, but a null value was found at index $index"
+    );
     if (addRepaintBoundaries)
       child = RepaintBoundary(child: child);
     if (addSemanticIndexes) {