refactor(@angular-devkit/schematics): allow rule to return void or rule promise

This allows a rule to conditionally not return a followup rule when asynchronous.  This is useful to handle error conditions where no further action should be taken and returning void best expresses this case.

Author: clydin

etc/api/angular_devkit/schematics/src/_golden-api.d.ts

@@ -419,7 +419,7 @@ export interface RequiredWorkflowExecutionContext {
     schematic: string;
 }
 
-export declare type Rule = (tree: Tree, context: SchematicContext) => Tree | Observable<Tree> | Rule | Promise<void> | Promise<Rule> | void;
+export declare type Rule = (tree: Tree, context: SchematicContext) => Tree | Observable<Tree> | Rule | Promise<void | Rule> | void;
 
 export declare type RuleFactory<T extends object> = (options: T) => Rule;
 

packages/angular_devkit/schematics/src/engine/interface.ts

@@ -227,4 +227,4 @@ export type AsyncFileOperator = (tree: FileEntry) => Observable<FileEntry | null
  */
 export type Source = (context: SchematicContext) => Tree | Observable<Tree>;
 export type Rule = (tree: Tree, context: SchematicContext) =>
-  Tree | Observable<Tree> | Rule | Promise<void> | Promise<Rule> | void;
+  Tree | Observable<Tree> | Rule | Promise<void | Rule> | void;