Add utility function resolveUrl on manager

packages/base/src/manager-base.ts

@@ -481,6 +481,16 @@ abstract class ManagerBase<T> {
             this._models[i].then(model => { model.comm_live = false; });
         });
     }
+
+    /**
+     * Resolve a URL relative to the current notebook location.
+     *
+     * The default implementation just returns the original url.
+     */
+    resolveUrl(url: string): Promise<string> {
+        return Promise.resolve(url);
+    }
+
     /**
      * The comm target name to register
      */

packages/jupyterlab-manager/src/manager.ts

@@ -156,6 +156,13 @@ class WidgetManager extends ManagerBase<Widget> implements IDisposable {
     this._context = null;
   }
 
+  /**
+   * Resolve a URL relative to the current notebook location.
+   */
+  resolveUrl(url: string): Promise<string> {
+    return this.context.resolveUrl(url);
+  }
+
   /**
    * Load a class and return a promise to the loaded object.
    */