FlexibleSpaceBar should not include transparent widgets (#3527)

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

@@ -80,12 +80,14 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
         curve: new Interval(math.max(0.0, fadeStart), math.min(fadeEnd, 1.0))
       );
       final double parallax = new Tween<double>(begin: 0.0, end: appBarHeight / 4.0).evaluate(_scaffoldAnimation);
+      final double opacity = new Tween<double>(begin: 1.0, end: 0.0).evaluate(opacityCurve);
+      if (opacity > 0.0) {
         children.add(new Positioned(
           top: -parallax,
           left: 0.0,
           right: 0.0,
           child: new Opacity(
-          opacity: new Tween<double>(begin: 1.0, end: 0.0).evaluate(opacityCurve),
+            opacity: opacity,
             child: new SizedBox(
               height: appBarHeight + statusBarHeight,
               child: config.background
@@ -93,6 +95,7 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
           )
         ));
       }
+    }
 
     // title
     if (config.title != null) {
@@ -102,9 +105,11 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
         parent: _scaffoldAnimation,
         curve: new Interval(fadeStart, fadeEnd)
       );
+      final int alpha = new Tween<double>(begin: 255.0, end: 0.0).evaluate(opacityCurve).toInt();
+      if (alpha > 0) {
         TextStyle titleStyle = Theme.of(context).primaryTextTheme.title;
         titleStyle = titleStyle.copyWith(
-        color: titleStyle.color.withAlpha(new Tween<double>(begin: 255.0, end: 0.0).evaluate(opacityCurve).toInt())
+          color: titleStyle.color.withAlpha(alpha)
         );
         final double yAlignStart = 1.0;
         final double yAlignEnd = (statusBarHeight + kToolBarHeight / 2.0) / toolBarHeight;
@@ -131,6 +136,7 @@ class _FlexibleSpaceBarState extends State<FlexibleSpaceBar> {
           )
         ));
       }
+    }
 
     return new ClipRect(child: new Stack(children: children));
   }