Sky: Add a CustomDart attribute to the IDL compiler, and use that with Canvas

sky/engine/core/painting/Canvas.dart

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// This extends the Canvas.dart generated from Canvas.idl.
+
+class Rect {
+  List<double> _value;
+  double get left => _value[0];
+  double get top => _value[1];
+  double get right => _value[2];
+  double get bottom => _value[3];
+
+  void setLTRB(double left, double top, double right, double bottom) {
+    _value = [left, top, right, bottom];
+  }
+}
+
+class Canvas extends _Canvas {
+  double get height => _height;
+  double get width => _width;
+
+  void save() => _save();
+  void saveLayer(Rect bounds, Paint paint) =>
+      _saveLayer(bounds._value, Paint paint);
+  void restore() => _restore();
+  void translate(double dx, double dy) => _translate(dx, dy);
+  void scale(double sx, double sy) => _scale(sx, sy);
+  void rotateDegrees(double degrees) => _rotateDegrees(degrees);
+  void skew(double sx, double sy) => _skew(sx, sy);
+  void concat(List<double> matrix9) => _concat(matrix9);
+  void clipRect(Rect rect) => _clipRect(rect._value);
+
+  void drawPicture(Picture picture) => _drawPicture(picture);
+  void drawPaint(Paint paint) => _drawPaint(paint);
+  void drawRect(Rect rect, Paint paint) => _drawRect(rect._value, paint);
+  void drawOval(Rect rect, Paint paint) => _drawOval(rect._value, paint);
+  void drawCircle(double x, double y, double radius, Paint paint) =>
+      _drawCircle(x, y, radius, paint);
+}