Add missing includes (spotted while splitting cpp derived sources).

client/dom/idl/dart/dart.idl

Index: client/dom/idl/dart/dart.idl
diff --git a/client/dom/idl/dart/dart.idl b/client/dom/idl/dart/dart.idl
index f7d7185ae5f947293fdbd032f47a65f1fb4e9708..d629420a5244eff38bfa7c650c9ccb57fa17e8f2 100644
--- a/client/dom/idl/dart/dart.idl
+++ b/client/dom/idl/dart/dart.idl
@@ -23,6 +23,14 @@ module default {
   // Uint8ClampedArray implements sequence<int>;
 }
 
+module core {
+  [Supplemental]
+  interface Document {
+    [Suppressed] object getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
+    CanvasRenderingContext getCSSCanvasContext(in DOMString contextId, in DOMString name, in long width, in long height);
+  };
+};
+
 module dom {
   // Force NodeSelector. WebKit defines these operations directly.
   interface NodeSelector {