RATIONALE: Drugs used in chemotherapy, such as irinotecan hydrochloride, leucovorin calcium, and fluorouracil, work in different ways to stop the growth of tumor cells, either by killing the cells or by stopping them from dividing. Bevacizumab may stop the growth of colorectal cancer by blocking blood flow to the tumor. It is not yet known whether giving more than one drug (combination chemotherapy) is more effective when given with or without bevacizumab in treating patients with metastatic colorectal cancer.