Added Scrollable.scrollTo()

sky/framework/components/scrollable.dart

Index: sky/framework/components/scrollable.dart
diff --git a/sky/framework/components/scrollable.dart b/sky/framework/components/scrollable.dart
index 86b774b4376cddfa98591143a0b3daa7034f5b87..0bbe0e55c4eb6ae20fc64300f935a4befd7c2448 100644
--- a/sky/framework/components/scrollable.dart
+++ b/sky/framework/components/scrollable.dart
@@ -43,8 +43,7 @@ abstract class Scrollable extends Component {
     );
   }
 
-  bool scrollBy(double scrollDelta) {
-    var newScrollOffset = scrollBehavior.applyCurve(_scrollOffset, scrollDelta);
+  bool scrollTo(double newScrollOffset) {
     if (newScrollOffset == _scrollOffset)
       return false;
     setState(() {
@@ -53,6 +52,11 @@ abstract class Scrollable extends Component {
     return true;
   }
 
+  bool scrollBy(double scrollDelta) {
+    var newScrollOffset = scrollBehavior.applyCurve(_scrollOffset, scrollDelta);
+    return scrollTo(newScrollOffset);
+  }
+
   void _stopSimulation() {
     if (_simulation == null)
       return;