LibJS: Make eval() return the last value from the executed statement

This is kinda awkward but since the statement we're executing is
actually a JS::Program, we have to get the result via VM::last_value().

Userland/Libraries/LibJS/Runtime/GlobalObject.cpp

@@ -325,8 +325,10 @@ JS_DEFINE_NATIVE_FUNCTION(GlobalObject::eval)
     auto& caller_frame = vm.call_stack().at(vm.call_stack().size() - 2);
     TemporaryChange scope_change(vm.call_frame().scope, caller_frame->scope);
 
-    // FIXME: eval() should return the result of the executed code. This currently does not work.
-    return vm.interpreter().execute_statement(global_object, program);
+    vm.interpreter().execute_statement(global_object, program);
+    if (vm.exception())
+        return {};
+    return vm.last_value();
 }
 
 }

Userland/Libraries/LibJS/Tests/eval-basic.js

@@ -0,0 +1,10 @@
+test("basic eval() functionality", () => {
+    expect(eval("1 + 2")).toBe(3);
+
+    function foo(a) {
+        var x = 5;
+        eval("x += a");
+        return x;
+    }
+    expect(foo(7)).toBe(12);
+});