Implement 'stretch' for flexible items.

packages/flutter/lib/rendering/flex.dart

@@ -341,6 +341,22 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
       if (flex > 0) {
         double spaceForChild = spacePerFlex * flex;
         BoxConstraints innerConstraints;
+        if (alignItems == FlexAlignItems.stretch) {
+          switch (_direction) {
+            case FlexDirection.horizontal:
+              innerConstraints = new BoxConstraints(minWidth: constraints.maxWidth,
+                                                    maxWidth: constraints.maxWidth,
+                                                    minHeight: constraints.minHeight,
+                                                    maxHeight: constraints.maxHeight);
+              break;
+            case FlexDirection.vertical:
+              innerConstraints = new BoxConstraints(minWidth: constraints.minWidth,
+                                                    maxWidth: constraints.maxWidth,
+                                                    minHeight: constraints.maxHeight,
+                                                    maxHeight: constraints.maxHeight);
+              break;
+          }
+        } else {
           switch (_direction) {
             case FlexDirection.horizontal:
               innerConstraints = new BoxConstraints(maxHeight: constraints.maxHeight,
@@ -353,6 +369,7 @@ class RenderFlex extends RenderBox with ContainerRenderObjectMixin<RenderBox, Fl
                                                     maxWidth: constraints.maxWidth);
               break;
           }
+        }
         child.layout(innerConstraints, parentUsesSize: true);
         usedSpace += _getMainSize(child);
         crossSize = math.max(crossSize, _getCrossSize(child));