Add black and white constants to theme's colors.

They're in the Material design color palette. This also adds a file-level
dart doc comment.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1190413003.

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),