LibJS: Add support for async functions

This commit adds support for the most bare bones version of async
functions, support for async generator functions, async arrow functions
and await expressions are TODO.

Userland/Libraries/LibJS/Runtime/AsyncFunctionConstructor.cpp

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Interpreter.h>
+#include <LibJS/Runtime/AsyncFunctionConstructor.h>
+#include <LibJS/Runtime/ECMAScriptFunctionObject.h>
+#include <LibJS/Runtime/FunctionConstructor.h>
+#include <LibJS/Runtime/FunctionObject.h>
+#include <LibJS/Runtime/GlobalObject.h>
+
+namespace JS {
+
+AsyncFunctionConstructor::AsyncFunctionConstructor(GlobalObject& global_object)
+    : NativeFunction(vm().names.AsyncFunction.as_string(), *global_object.function_prototype())
+{
+}
+
+void AsyncFunctionConstructor::initialize(GlobalObject& global_object)
+{
+    auto& vm = this->vm();
+    NativeFunction::initialize(global_object);
+
+    // 27.7.2.2 AsyncFunction.prototype, https://tc39.es/ecma262/#sec-async-function-constructor-prototype
+    define_direct_property(vm.names.prototype, global_object.async_function_prototype(), 0);
+
+    define_direct_property(vm.names.length, Value(1), Attribute::Configurable);
+}
+
+// 27.7.1.1 AsyncFunction ( p1, p2, … , pn, body ), https://tc39.es/ecma262/#sec-async-function-constructor-arguments
+ThrowCompletionOr<Value> AsyncFunctionConstructor::call()
+{
+    return TRY(construct(*this));
+}
+
+// 27.7.1.1 AsyncFunction ( p1, p2, … , pn, body ), https://tc39.es/ecma262/#sec-async-function-constructor-arguments
+ThrowCompletionOr<Object*> AsyncFunctionConstructor::construct(FunctionObject& new_target)
+{
+    auto& vm = this->vm();
+    auto function = TRY(FunctionConstructor::create_dynamic_function_node(global_object(), new_target, FunctionKind::Async));
+
+    OwnPtr<Interpreter> local_interpreter;
+    Interpreter* interpreter = vm.interpreter_if_exists();
+
+    if (!interpreter) {
+        local_interpreter = Interpreter::create_with_existing_realm(*realm());
+        interpreter = local_interpreter.ptr();
+    }
+
+    VM::InterpreterExecutionScope scope(*interpreter);
+    auto result = function->execute(*interpreter, global_object());
+    if (auto* exception = vm.exception())
+        return throw_completion(exception->value());
+    VERIFY(result.is_object() && is<ECMAScriptFunctionObject>(result.as_object()));
+    return &result.as_object();
+}
+
+}