Epirubicin Hydrochloride, Cisplatin, and Fluorouracil or Capecitabine With or Without Lapatinib Ditosylate as First-Line Therapy in Treating Patients With Stomach Cancer or Gastroesophageal Junction Cancer

RATIONALE: Drugs used in chemotherapy, such as epirubicin hydrochloride, cisplatin, fluorouracil, and capecitabine, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Lapatinib ditosylate may stop the growth of tumor cells by blocking some of the enzymes needed for cell growth. It is not yet known whether giving combination chemotherapy together with or without lapatinib ditosylate is more effective in treating patients with cancer of the stomach or gastroesophageal junction. PURPOSE: This randomized phase II trial is studying how well epirubicin hydrochloride, cisplatin, and fluorouracil or capecitabine works when given together with or without lapatinib ditosylate as first-line therapy in treating patients with stomach cancer or gastroesophageal junction cancer.