Add Iterable.fold (and Stream.fold) which replace `reduce`.

samples/openglui/src/openglui_canvas_tests.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 //
 // Canvas API tests. Some of these are adapted from:
 //
 // http://www.html5canvastutorials.com/
 
 library openglui_canvas_tests;
 
@@ skipping 1004 matching lines @@
                                  : Color.defaultColor;
         var specular = Vector.dot(livec, Vector.norm(rd));
         var scolor = (specular > 0)
             ? Color.scale(Math.pow(specular, thing.surface.roughness),
                 light.color)
             : Color.defaultColor;
         return col + (thing.surface.diffuse(pos)  * lcolor)
             + (thing.surface.specular(pos) * scolor);
       }
     };
-    return scene.lights.reduce(Color.defaultColor, addLight);
+    return scene.lights.fold(Color.defaultColor, addLight);
   }
 
   render(Scene scene, CanvasRenderingContext2D ctx, num screenWidth,
          num screenHeight) {
     var getPoint = (x, y, camera) {
       var recenterX = (x) => (x - (screenWidth / 2.0)) / 2.0 / screenWidth;
       var recenterY = (y) => - (y - (screenHeight / 2.0)) / 2.0 / screenHeight;
       return Vector.norm(camera.forward
           + Vector.times(recenterX(x), camera.right)
           + Vector.times(recenterY(y), camera.up));
     };
     for (int y = 0; y < screenHeight; y++) {
       for (int x = 0; x < screenWidth; x++) {
         var color = _traceRay(new Ray(scene.camera.pos,
             getPoint(x, y, scene.camera) ), scene, 0);
         ctx.fillStyle = Color.toDrawingRGB(color);
         ctx.fillRect(x, y, x + 1, y + 1);
       }
     }
   }
 }