Specs: Simplify the paint model. Now you are not responsible for

actually telling your child to paint, you just say where it would
paint. The platform then takes care of making sure all the dirty nodes
have their paint() methods called.

Review URL: https://codereview.chromium.org/744843003

examples/style/hex-layout.sky

@@ -69,7 +69,6 @@
      let loop = children.next();
      while (!loop.done) {
        let child = loop.value;
-       if (child.needsPaint || child.descendantNeedsPaint) {
        canvas.save();
        try {
          canvas.beginPath();
@@ -81,11 +80,10 @@
          canvas.moveTo(child.x, child.y + 3*cellDim/4);
          canvas.closePath();
          canvas.clip();
-           this.paintChild(child);
+         canvas.paintChild(child);
        } finally {
          canvas.restore();
        }
-       }
        loop = children.next();
      }
    }

examples/style/toolbar-layout.sky

@@ -188,9 +188,9 @@ SKY MODULE
      let children = this.walkChildren();
      let loop = children.next();
      while ((!loop.done) && (loop.value != this.firstSkippedChild))
-       this.paintChild(loop.value, canvas);
+       canvas.paintChild(loop.value);
      if (this.showingOverflow)
-       this.paintChild(this.overflowChild, canvas);
+       canvas.paintChild(this.overflowChild);
    }
    function inChild(child, x, y) {
      return (x >= child.x) && (y >= child.y) && (x < child.x+child.width) && (y < child.y+child.height);