Aggressively try to catch misuse of DropDownButton

Fixes https://github.com/flutter/flutter/issues/948.
I hope.

packages/flutter/lib/src/material/dropdown.dart

@@ -209,7 +209,9 @@ class DropDownButton<T> extends StatefulComponent {
     this.value,
     this.onChanged,
     this.elevation: 8
-  }) : super(key: key);
+  }) : super(key: key) {
+    assert(items.where((DropDownMenuItem<T> item) => item.value == value).length == 1);
+  }
 
   final List<DropDownMenuItem<T>> items;
   final T value;
@@ -225,6 +227,7 @@ class _DropDownButtonState<T> extends State<DropDownButton<T>> {
   void initState() {
     super.initState();
     _updateSelectedIndex();
+    assert(_selectedIndex != null);
   }
 
   void didUpdateConfig(DropDownButton<T> oldConfig) {

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

@@ -433,7 +433,8 @@ class RenderStack extends RenderStackBase {
 
 /// Implements the same layout algorithm as RenderStack but only paints the child
 /// specified by index.
-/// Note: although only one child is displayed, the cost of the layout algorithm is
+///
+/// Although only one child is displayed, the cost of the layout algorithm is
 /// still O(N), like an ordinary stack.
 class RenderIndexedStack extends RenderStackBase {
   RenderIndexedStack({
@@ -443,11 +444,14 @@ class RenderIndexedStack extends RenderStackBase {
   }) : _index = index, super(
    children: children,
    alignment: alignment
-  );
+  ) {
+    assert(index != null);
+  }
 
   int get index => _index;
   int _index;
   void set index (int value) {
+    assert(value != null);
     if (_index != value) {
       _index = value;
       markNeedsLayout();

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

@@ -962,7 +962,9 @@ class IndexedStack extends MultiChildRenderObjectWidget {
     Key key,
     this.alignment: const FractionalOffset(0.0, 0.0),
     this.index: 0
-  }) : super(key: key, children: children);
+  }) : super(key: key, children: children) {
+    assert(index != null);
+  }
 
   /// The index of the child to show.
   final int index;