reverting logic to determine serverBasePath

Author: legrego

x-pack/plugins/spaces/public/plugin.tsx

@@ -43,7 +43,7 @@ export class SpacesPlugin implements Plugin<SpacesPluginSetup, SpacesPluginStart
   private managementService?: ManagementService;
 
   public setup(core: CoreSetup, plugins: PluginsSetup) {
-    const serverBasePath = core.http.basePath.get();
+    const serverBasePath = core.injectedMetadata.getInjectedVar('serverBasePath') as string;
     this.spacesManager = new SpacesManager(serverBasePath, core.http);
 
     if (plugins.home) {