Material light and dark themes for Sky widgets

sky/sdk/lib/painting/text_style.dart

Index: sky/sdk/lib/painting/text_style.dart
diff --git a/sky/sdk/lib/painting/text_style.dart b/sky/sdk/lib/painting/text_style.dart
index 19d3c547e0447c9560d92de91ce36220cb91490f..cf8a7422e936b1ef02746941c1e302a9001d4b56 100644
--- a/sky/sdk/lib/painting/text_style.dart
+++ b/sky/sdk/lib/painting/text_style.dart
@@ -74,6 +74,7 @@ class TextStyle {
 
   TextStyle copyWith({
     Color color,
+    String fontFamily,
     double fontSize,
     FontWeight fontWeight,
     TextAlign textAlign,
@@ -97,6 +98,19 @@ class TextStyle {
     return 'rgba(${color.red}, ${color.green}, ${color.blue}, ${color.alpha / 255.0})';
   }
 
+  TextStyle merge(TextStyle other) {
+    return copyWith(
+      color: other.color,
+      fontFamily: other.fontFamily,
+      fontSize: other.fontSize,
+      fontWeight: other.fontWeight,
+      textAlign: other.textAlign,
+      decoration: other.decoration,
+      decorationColor: other.decorationColor,
+      decorationStyle: other.decorationStyle
+    );
+  }
+
   static String _fontFamilyToCSSString(String fontFamily) {
     // TODO(hansmuller): escape the fontFamily string.
     return fontFamily;