Adds API docs to sprite ColorSequence and Layer

examples/game/lib/color_secuence.dart

 part of sprites;
 
+/// A sequence of colors representing a gradient or a color transition over
+/// time. The sequence is represented by a list of [colors] and a list of
+/// [colorStops], the stops are normalized values (0.0 to 1.0) and ordered in
+/// the list. Both lists have the same number of elements.
 class ColorSequence {
+  /// List of colors.
   List<Color> colors;
+
+  /// List of color stops, normalized values (0.0 to 1.0) and ordered.
   List<double> colorStops;
 
+  /// Creates a new color sequence from a list of [colors] and a list of
+  /// [colorStops].
   ColorSequence(this.colors, this.colorStops) {
     assert(colors != null);
     assert(colorStops != null);
     assert(colors.length == colorStops.length);
   }
 
+  /// Creates a new color sequence from a start and an end color.
   ColorSequence.fromStartAndEndColor(Color start, Color end) {
     colors = [start, end];
     colorStops = [0.0, 1.0];
   }
 
+  /// Creates a new color sequence by copying an existing sequence.
   ColorSequence.copy(ColorSequence sequence) {
     colors = new List<Color>.from(sequence.colors);
     colorStops = new List<double>.from(sequence.colorStops);
   }
 
+  /// Returns the color at a normalized (0.0 to 1.0) position in the color
+  /// sequence. If a color stop isn't hit, the returned color will be an
+  /// interpolation of a color between two color stops.
   Color colorAtPosition(double pos) {
     assert(pos >= 0.0 && pos <= 1.0);
 

examples/game/lib/layer.dart

 part of sprites;
 
+/// A [Node] that provides an intermediate rendering surface in the sprite
+/// rendering tree. A [Layer] can be used to change the opacity, color, or to
+/// apply an effect to a set of nodes. All nodes that are children to the
+/// [Layer] will be rendered into the surface. If the area that is needed for
+/// the children to be drawn is know, the [layerRect] property should be set as
+/// this can enhance performance.
 class Layer extends Node with SpritePaint {
+
+  /// The area that the children of the [Layer] will occupy. This value is
+  /// treated as a hint to the rendering system and may in some cases be
+  /// ignored. If the area isn't known, the layerRect can be set to [null].
+  ///
+  ///     myLayer.layerRect = new Rect.fromLTRB(0.0, 0.0, 200.0, 100.0);
   Rect layerRect;
 
+  /// Creates a new layer. The layerRect can optionally be passed as an argument
+  /// if it is known.
+  ///
+  ///     var myLayer = new Layer();
   Layer([Rect this.layerRect = null]);
 
   Paint _cachedPaint = new Paint()