Tab bar should have white indicator if primary and accent color are the same

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1215313004 .

sdk/lib/widgets/tabs.dart

@@ -11,6 +11,7 @@ import 'package:sky/painting/text_style.dart';
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/object.dart';
 import 'package:vector_math/vector_math.dart';
+import 'package:sky/theme/colors.dart' as colors;
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/icon.dart';
 import 'package:sky/widgets/ink_well.dart';
@@ -433,11 +434,17 @@ class TabBar extends Scrollable {
         textAndIcons = true;
     }
 
+    Color backgroundColor = Theme.of(this).primaryColor;
+    Color indicatorColor = Theme.of(this).accentColor;
+    if (indicatorColor == backgroundColor) {
+      indicatorColor = colors.White;
+    }
+
     TabBarWrapper tabBarWrapper = new TabBarWrapper(
       children: tabs, 
       selectedIndex: selectedIndex,
-      backgroundColor: Theme.of(this).primaryColor,
-      indicatorColor: Theme.of(this).accentColor,
+      backgroundColor: backgroundColor,
+      indicatorColor: indicatorColor,
       textAndIcons: textAndIcons,
       scrollable: scrollable,
       onLayoutChanged: scrollable ? _layoutChanged : null

tests/examples/stocks-expected.txt

@@ -27,7 +27,7 @@ PAINT FOR FRAME #2 ----------------------------------------------
 2 |  |  |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 2 |  |  |  |  |  |  |  | translate(171.0, 14.0)
 2 |  |  |  |  |  |  |  | translate(-171.0, -14.0)
-2 |  |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 400.0, 48.0), Paint(color:Color(0xff9c27b0)))
+2 |  |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 400.0, 48.0), Paint(color:Color(0xffffffff)))
 2 |  |  |  |  | paintChild RenderInkWell at Point(400.0, 0.0)
 2 |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 2 |  |  |  |  |  | paintChild RenderPositionedBox at Point(412.0, 0.0)
@@ -107,7 +107,7 @@ PAINT FOR FRAME #3 ----------------------------------------------
 3 |  |  |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 3 |  |  |  |  |  |  |  | translate(171.0, 14.0)
 3 |  |  |  |  |  |  |  | translate(-171.0, -14.0)
-3 |  |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 400.0, 48.0), Paint(color:Color(0xff9c27b0)))
+3 |  |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 400.0, 48.0), Paint(color:Color(0xffffffff)))
 3 |  |  |  |  | paintChild RenderInkWell at Point(400.0, 0.0)
 3 |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 3 |  |  |  |  |  | paintChild RenderPositionedBox at Point(412.0, 0.0)

tests/examples/tabs-expected.txt

@@ -25,7 +25,7 @@ PAINT FOR FRAME #2 ----------------------------------------------
 2 |  |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 2 |  |  |  |  |  |  | translate(82.0, 14.0)
 2 |  |  |  |  |  |  | translate(-82.0, -14.0)
-2 |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 200.0, 48.0), Paint(color:Color(0xff2196f3)))
+2 |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 200.0, 48.0), Paint(color:Color(0xffffffff)))
 2 |  |  |  | paintChild RenderInkWell at Point(200.0, 0.0)
 2 |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 2 |  |  |  |  | paintChild RenderPositionedBox at Point(212.0, 0.0)
@@ -84,7 +84,7 @@ PAINT FOR FRAME #2 ----------------------------------------------
 2 |  |  |  |  |  |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 2 |  |  |  |  |  |  |  |  |  |  | translate(78.5, 14.0)
 2 |  |  |  |  |  |  |  |  |  |  | translate(-78.5, -14.0)
-2 |  |  |  |  |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 188.0, 48.0), Paint(color:Color(0xff2196f3)))
+2 |  |  |  |  |  |  |  | drawRect(Rect.fromLTRB(0.0, 46.0, 188.0, 48.0), Paint(color:Color(0xffffffff)))
 2 |  |  |  |  |  |  |  | paintChild RenderInkWell at Point(188.0, 0.0)
 2 |  |  |  |  |  |  |  |  | TestPaintingCanvas() constructor: 800.0 x 600.0
 2 |  |  |  |  |  |  |  |  | paintChild RenderPositionedBox at Point(200.0, 0.0)