Factor ScrollCurve out of Scrollable

Intead of hard-coding the notion of bounded scrolling into Scrollable, this CL
factors out a ScrollCurve class that applies the bounds. In the future, we'll
refine this mechanism to implement overflow scrolling.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1005633002

examples/stocks-fn/stocklist.dart

@@ -8,7 +8,7 @@ class Stocklist extends FixedHeightScrollable {
     Object key,
     this.stocks,
     this.query
-  }) : super(key: key, minOffset: 0.0);
+  }) : super(key: key, scrollCurve: new BoundedScrollCurve(minOffset: 0.0));
 
   List<Node> buildItems(int start, int count) {
     return stocks

examples/stocks-fn/stocksapp.dart

 library stocksapp;
 
 import '../../framework/fn.dart';
+import '../../framework/animation/scroll_curve.dart';
 import '../../framework/components/drawer.dart';
 import '../../framework/components/drawer_header.dart';
 import '../../framework/components/fixed_height_scrollable.dart';