fix(@angular-devkit/schematics): fully support async rules

Author: clydin

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

@@ -426,7 +426,7 @@ export interface RequiredWorkflowExecutionContext {
     schematic: string;
 }
 
-export declare type Rule = (tree: Tree, context: SchematicContext) => Tree | Observable<Tree> | Rule | Promise<void> | void;
+export declare type Rule = (tree: Tree, context: SchematicContext) => Tree | Observable<Tree> | Rule | Promise<void> | Promise<Rule> | void;
 
 export declare type RuleFactory<T extends object> = (options: T) => Rule;
 

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

@@ -231,4 +231,4 @@ 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> | void;
+  Tree | Observable<Tree> | Rule | Promise<void> | Promise<Rule> | void;

packages/angular_devkit/schematics/src/rules/call.ts

@@ -97,7 +97,16 @@ export function callRule(
         }),
       );
     } else if (isPromise(result)) {
-      return from(result).pipe(map(() => inputTree));
+      return from(result).pipe(
+        mergeMap(inner => {
+          if (typeof inner === 'function') {
+            // This is considered a Rule, chain the rule and return its output.
+            return callRule(inner, observableOf(inputTree), context);
+          } else {
+            return observableOf(inputTree);
+          }
+        }),
+      );
     } else if (TreeSymbol in result) {
       return observableOf(result);
     } else {