A few minor improvements

This patch contains a few minor improvements to the framework that I ran
across while working on a new layout model.

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

@@ -1049,7 +1049,7 @@ abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, Pare
   /// appear in the child list.
   double defaultComputeDistanceToFirstActualBaseline(TextBaseline baseline) {
     assert(!needsLayout);
-    RenderBox child = firstChild;
+    ChildType child = firstChild;
     while (child != null) {
       final ParentDataType childParentData = child.parentData;
       double result = child.getDistanceToActualBaseline(baseline);
@@ -1067,7 +1067,7 @@ abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, Pare
   double defaultComputeDistanceToHighestActualBaseline(TextBaseline baseline) {
     assert(!needsLayout);
     double result;
-    RenderBox child = firstChild;
+    ChildType child = firstChild;
     while (child != null) {
       final ParentDataType childParentData = child.parentData;
       double candidate = child.getDistanceToActualBaseline(baseline);
@@ -1103,13 +1103,24 @@ abstract class RenderBoxContainerDefaultsMixin<ChildType extends RenderBox, Pare
 
   /// Paints each child by walking the child list forwards.
   void defaultPaint(PaintingContext context, Offset offset) {
-    RenderBox child = firstChild;
+    ChildType child = firstChild;
     while (child != null) {
       final ParentDataType childParentData = child.parentData;
       context.paintChild(child, childParentData.offset + offset);
       child = childParentData.nextSibling;
     }
   }
+
+  List<ChildType> getChildrenAsList() {
+    List<ChildType> result = <ChildType>[];
+    RenderBox child = firstChild;
+    while (child != null) {
+      final ParentDataType childParentData = child.parentData;
+      result.add(child);
+      child = childParentData.nextSibling;
+    }
+    return result;
+  }
 }
 
 class FractionalOffsetTween extends Tween<FractionalOffset> {

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

@@ -1118,7 +1118,7 @@ class RenderTransform extends RenderProxyBox {
   /// The alignment of the origin, relative to the size of the box.
   ///
   /// This is equivalent to setting an origin based on the size of the box.
-  /// If it is specificed at the same time as an offset, both are applied.
+  /// If it is specified at the same time as an offset, both are applied.
   FractionalOffset get alignment => _alignment;
   FractionalOffset _alignment;
   void set alignment (FractionalOffset newAlignment) {

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

@@ -2570,6 +2570,13 @@ class RepaintBoundary extends SingleChildRenderObjectWidget {
     return new RepaintBoundary(key: key, child: child);
   }
 
+  static List<RepaintBoundary> wrapAll(List<Widget> widgets) {
+    List<RepaintBoundary> result = new List<RepaintBoundary>(widgets.length);
+    for (int i = 0; i < result.length; ++i)
+      result[i] = new RepaintBoundary.wrap(widgets[i], i);
+    return result;
+  }
+
   @override
   RenderRepaintBoundary createRenderObject(BuildContext context) => new RenderRepaintBoundary();
 }