LibJS: Implement Iterator.from and the WrapForValidIteratorPrototype

Iterator.from creates an Iterator from either an existing iterator or
an iterator-like object. In the latter case, it sets the prototype of
the returned iterator to WrapForValidIteratorPrototype to wrap around
the iterator-like object's iteration methods.

Userland/Libraries/LibJS/Runtime/Iterator.cpp

@@ -4,7 +4,9 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <LibJS/Runtime/AbstractOperations.h>
 #include <LibJS/Runtime/Iterator.h>
+#include <LibJS/Runtime/VM.h>
 
 namespace JS {
 
@@ -24,4 +26,47 @@ Iterator::Iterator(Object& prototype)
 {
 }
 
+// 2.1.1 GetIteratorDirect ( obj ), https://tc39.es/proposal-iterator-helpers/#sec-getiteratorflattenable
+ThrowCompletionOr<IteratorRecord> get_iterator_direct(VM& vm, Object& object)
+{
+    // 1. Let nextMethod be ? Get(obj, "next").
+    auto next_method = TRY(object.get(vm.names.next));
+
+    // 2. Let iteratorRecord be Record { [[Iterator]]: obj, [[NextMethod]]: nextMethod, [[Done]]: false }.
+    IteratorRecord iterator_record { .iterator = object, .next_method = next_method, .done = false };
+
+    // 3. Return iteratorRecord.
+    return iterator_record;
+}
+
+ThrowCompletionOr<IteratorRecord> get_iterator_flattenable(VM& vm, Value object)
+{
+    // 1. If obj is not an Object, throw a TypeError exception.
+    if (!object.is_object())
+        return vm.throw_completion<TypeError>(ErrorType::NotAnObject, "obj"sv);
+
+    // 2. Let method be ? GetMethod(obj, @@iterator).
+    auto method = TRY(object.get_method(vm, vm.well_known_symbol_iterator()));
+
+    Value iterator;
+
+    // 3. If method is undefined, then
+    if (!method) {
+        // a. Let iterator be obj.
+        iterator = object;
+    }
+    // 4. Else,
+    else {
+        // a. Let iterator be ? Call(method, obj).
+        iterator = TRY(call(vm, method, object));
+    }
+
+    // 5. If iterator is not an Object, throw a TypeError exception.
+    if (!iterator.is_object())
+        return vm.throw_completion<TypeError>(ErrorType::NotAnObject, "iterator"sv);
+
+    // 6. Return ? GetIteratorDirect(iterator).
+    return TRY(get_iterator_direct(vm, iterator.as_object()));
+}
+
 }