stocks and stocks-fn should share the stocks data

Previously, the two versions of this sample app had separate copies of the
stock data. This CL moves the common data to the common data directory so that
they can share. Also, I've switched the data files to be |dart| files rather
than |sky| files to make them easier to use from fn examples.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/980953002

engine/src/flutter/examples/scrolling/scrolling.sky

@@ -5,7 +5,6 @@
 -->
 <import src="/sky/framework/sky-element.sky" />
 <import src="/sky/framework/sky-scrollable.sky" />
-<import src="../data/cities.sky" as="cities" />
 
 <sky-element>
 <template>
@@ -20,6 +19,7 @@
 </template>
 <script>
 import "dart:sky";
+import "../data/cities.dart" as cities;
 
 @Tagname('scrolling')
 class Scrolling extends SkyElement {