Agree Wige, sandbox or sandboxing is a liberal term for any closed environment, in programming usually a testing environment, for example PayPal has a developer sandbox for testing as do most webservices. I do a a lot of work with bank webservices handling application and approval data etc they all by necessity provide a testing area usually calling it The Sandbox.

OP has a very good point re clients. Sandbox will mean whatever Google tells them it means, although you have to doubt the larger web audience cares which browser they use, as seen by older version IE usage.