improve divider appearance in dark mode

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1212343011.

sdk/example/stocks/lib/stock_row.dart

@@ -53,9 +53,9 @@ class StockRow extends Component {
       child: new Container(
         padding: const EdgeDims(16.0, 16.0, 20.0, 16.0),
         height: kHeight,
-        decoration: const BoxDecoration(
-          border: const Border(
-            bottom: const BorderSide(color: const Color(0xFFF4F4F4))
+        decoration: new BoxDecoration(
+          border: new Border(
+            bottom: new BorderSide(color: Theme.of(this).dividerColor)
           )
         ),
         child: new Flex(children)

sdk/lib/theme/theme_data.dart

@@ -22,6 +22,7 @@ class ThemeData {
       this.primarySwatch = primarySwatch,
       canvasColor = brightness == ThemeBrightness.dark ? colors.Grey[850] : colors.Grey[50],
       cardColor = brightness == ThemeBrightness.dark ? colors.Grey[800] : colors.White,
+      dividerColor = brightness == ThemeBrightness.dark ? const Color(0x1FFFFFFF) : const Color(0x1F000000),
       text = brightness == ThemeBrightness.dark ? typography.white : typography.black {
     assert(brightness != null);
 
@@ -61,6 +62,7 @@ class ThemeData {
   final Map<int, Color> primarySwatch;
   final Color canvasColor;
   final Color cardColor;
+  final Color dividerColor;
   final typography.TextTheme text;
 
   Color _primaryColor;

sdk/lib/widgets/menu_divider.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'basic.dart';
+import 'theme.dart';
 
 class MenuDivider extends Component {
   MenuDivider({ String key }) : super(key: key);
@@ -10,10 +11,10 @@ class MenuDivider extends Component {
   Widget build() {
     return new Container(
       height: 0.0,
-      decoration: const BoxDecoration(
-        border: const Border(
-          bottom: const BorderSide(
-            color: const Color.fromARGB(31, 0, 0, 0)
+      decoration: new BoxDecoration(
+        border: new Border(
+          bottom: new BorderSide(
+            color: Theme.of(this).dividerColor
           )
         )
       ),