Extend Sky's RenderFlex with intrinsic sizes and compute cross-size height

Renders interactive_flex and stocks demos fine. Could use more testing,
so I'm going to work on that after I land this.

R=abarth@chromium.org, abarth

Review URL: https://codereview.chromium.org/1173493003

examples/lib/solid_color_box.dart

@@ -14,19 +14,27 @@ class RenderSolidColorBox extends RenderDecoratedBox {
         super(decoration: new BoxDecoration(backgroundColor: backgroundColor));
 
   double getMinIntrinsicWidth(BoxConstraints constraints) {
-    return constraints.constrainWidth(desiredSize.width);
+    return constraints.constrainHeight(
+      this.desiredSize == Size.infinite ? 0.0 : desiredSize.width
+    );
   }
 
   double getMaxIntrinsicWidth(BoxConstraints constraints) {
-    return constraints.constrainWidth(desiredSize.width);
+    return constraints.constrainWidth(
+      this.desiredSize == Size.infinite ? 0.0 : desiredSize.width
+    );
   }
 
   double getMinIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight(desiredSize.height);
+    return constraints.constrainHeight(
+      this.desiredSize == Size.infinite ? 0.0 : desiredSize.height
+    );
   }
 
   double getMaxIntrinsicHeight(BoxConstraints constraints) {
-    return constraints.constrainHeight(desiredSize.height);
+    return constraints.constrainHeight(
+      this.desiredSize == Size.infinite ? 0.0 : desiredSize.height
+    );
   }
 
   void performLayout() {

examples/stocks2/lib/stock_menu.dart

@@ -27,7 +27,7 @@ class StockMenu extends Component {
         items: [
           [new Text('Add stock')],
           [new Text('Remove stock')],
-          // [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
+          [new FlexExpandingChild(new Text('Autorefresh')), checkbox],
         ],
         level: 4
       ),