refactor(@angular-devkit/core): use more precise iterator typings

Since this class isn't a "real" `Set`, we can't promise that our
iterators are true `SetIterator<T>`s.

(cherry picked from commit 16ae68a)

Author: hybrist

goldens/public-api/angular_devkit/core/index.api.md

@@ -567,15 +567,15 @@ export class PartiallyOrderedSet<T> {
     clear(): void;
     // (undocumented)
     delete(item: T): boolean;
-    entries(): SetIterator<[T, T]>;
+    entries(): IterableIterator<[T, T]>;
     // (undocumented)
     forEach(callbackfn: (value: T, value2: T, set: PartiallyOrderedSet<T>) => void, thisArg?: any): void;
     // (undocumented)
     has(item: T): boolean;
-    keys(): SetIterator<T>;
+    keys(): IterableIterator<T>;
     // (undocumented)
     get size(): number;
-    values(): SetIterator<T>;
+    values(): IterableIterator<T>;
 }
 
 // @public

packages/angular_devkit/core/src/utils/partially-ordered-set.ts

@@ -51,7 +51,7 @@ export class PartiallyOrderedSet<T> {
   /**
    * Returns an iterable of [v,v] pairs for every value `v` in the set.
    */
-  *entries(): SetIterator<[T, T]> {
+  *entries(): IterableIterator<[T, T]> {
     for (const item of this) {
       yield [item, item];
     }
@@ -60,14 +60,14 @@ export class PartiallyOrderedSet<T> {
   /**
    * Despite its name, returns an iterable of the values in the set,
    */
-  keys(): SetIterator<T> {
+  keys(): IterableIterator<T> {
     return this.values();
   }
 
   /**
    * Returns an iterable of values in the set.
    */
-  values(): SetIterator<T> {
+  values(): IterableIterator<T> {
     return this[Symbol.iterator]();
   }