CAMBRIDGE - Even before its 2001 founding, the story of Momenta Pharmaceuticals Inc. perfectly showcased the Massachusetts biomedical start-up ecosystem at work. It was born in MIT labs, where scientists developed a breakthrough process for understanding the structure of complex sugars. It was funded by a high-powered investor group led by Waltham’s Polaris Venture Partners. And it tapped into the star power of Boston’s world-famous medical institutions to assemble its scientific advisory board.