Move sky/examples to sky/sdk/lib/example, and code changes to support that change. Fixes T277.

sky/examples/game/lib/image_map.dart

-part of sprites;
-
-typedef void ImageMapCallback(ImageMap preloader);
-
-class ImageMap {
-
-  Map<String, Image> _images;
-
-  int _totalNumImages = 0;
-  int _numLoadedImages = 0;
-
-  ImageMapCallback _callback;
-
-  ImageMap(List<String> urls, ImageMapCallback this._callback) {
-    _images = new Map();
-    _totalNumImages = urls.length;
-    urls.forEach(_addURL);
-  }
-
-  void _addURL(String url) {
-    image_cache.load(url, (Image image) {
-      // Store reference to image
-      _images[url] = image;
-
-      // Check if all images are loaded
-      _numLoadedImages++;
-      if (_numLoadedImages==_totalNumImages) {
-        // Everything loaded, make callback
-        _callback(this);
-      }
-    });
-  }
-
-  Image getImage(String url) => _images[url];
-
-  Image operator [](String url) => _images[url];
-}