Add black and white constants to theme's colors.

sky/sdk/lib/theme/colors.dart

Index: sky/sdk/lib/theme/colors.dart
diff --git a/sky/sdk/lib/theme/colors.dart b/sky/sdk/lib/theme/colors.dart
index 658ababcef09435df47b28035d32e28714345c18..c5f666bf65b2de3af9b2cb11d2520ab48763afbf 100644
--- a/sky/sdk/lib/theme/colors.dart
+++ b/sky/sdk/lib/theme/colors.dart
@@ -4,6 +4,12 @@
 
 import 'dart:sky' show Color;
 
+/// [Color] constants which represent Material design's
+/// [color palette](http://www.google.com/design/spec/style/color.html).
+
+const White = const Color(0xFFFFFFFF);
+const Black = const Color(0x00000000);
+
 const Map<int, Color> Red = const {
    50: const Color(0xFFFFEBEE),
   100: const Color(0xFFFFCDD2),