Add abstract tag at class level

src/ol/source/image.js

@@ -17,6 +17,7 @@ goog.require('ol.source.Source');
  * Base class for sources providing a single image.
  *
  * @constructor
+ * @abstract
  * @extends {ol.source.Source}
  * @param {ol.SourceImageOptions} options Single image source options.
  * @api

src/ol/source/source.js

@@ -16,6 +16,7 @@ goog.require('ol.source.State');
  * A generic `change` event is triggered when the state of the source changes.
  *
  * @constructor
+ * @abstract
  * @extends {ol.Object}
  * @param {ol.SourceSourceOptions} options Source options.
  * @api stable

src/ol/source/tile.js

@@ -18,6 +18,7 @@ goog.require('ol.tilegrid');
  * Base class for sources providing images divided into a tile grid.
  *
  * @constructor
+ * @abstract
  * @extends {ol.source.Source}
  * @param {ol.SourceTileOptions} options Tile source options.
  * @api

src/ol/source/urltile.js

@@ -12,6 +12,7 @@ goog.require('ol.source.TileEventType');
  * Base class for sources providing tiles divided into a tile grid over http.
  *
  * @constructor
+ * @abstract
  * @fires ol.source.Tile.Event
  * @extends {ol.source.Tile}
  * @param {ol.SourceUrlTileOptions} options Image tile options.