Add a Color class to dart:sky.

sky/engine/core/script/dom_dart_state.cc

Index: sky/engine/core/script/dom_dart_state.cc
diff --git a/sky/engine/core/script/dom_dart_state.cc b/sky/engine/core/script/dom_dart_state.cc
index 121bde044154f4a0c120b489870fd0bc554ca78c..86bebaf806e79f7450980b4679bcca6178928570 100644
--- a/sky/engine/core/script/dom_dart_state.cc
+++ b/sky/engine/core/script/dom_dart_state.cc
@@ -7,6 +7,7 @@
 
 #include "sky/engine/core/dom/Document.h"
 #include "sky/engine/core/script/dart_loader.h"
+#include "sky/engine/tonic/dart_builtin.h"
 
 namespace blink {
 
@@ -22,6 +23,9 @@ void DOMDartState::DidSetIsolate() {
   x_handle_.Set(this, Dart_NewStringFromCString("x"));
   y_handle_.Set(this, Dart_NewStringFromCString("y"));
   value_handle_.Set(this, Dart_NewStringFromCString("_value"));
+
+  Dart_Handle sky_library = DartBuiltin::LookupLibrary("dart:sky");
+  color_class_.Set(this, Dart_GetType(sky_library, ToDart("Color"), 0, 0));
 }
 
 DOMDartState* DOMDartState::Current() {