use color.shadeXxx instead of color[Xxx] (#8932)

* use color.shadeXxx instead of color[Xxx]

* remove calls to .shade500 on MaterialColor

* remove calls to .shade200 on MaterialAccentColor

* fix test
diff --git a/examples/stocks/lib/main.dart b/examples/stocks/lib/main.dart
index 8b78a60..1cbfd1a 100644
--- a/examples/stocks/lib/main.dart
+++ b/examples/stocks/lib/main.dart
@@ -72,7 +72,7 @@
       case StockMode.pessimistic:
         return new ThemeData(
           brightness: Brightness.dark,
-          accentColor: Colors.redAccent[200]
+          accentColor: Colors.redAccent
         );
     }
     assert(_configuration.stockMode != null);