typechecker: support stdClass

Summary: stdClass is a special builtin in that it's basically designed
 for dynamic properties. So, special case 'stdClass' in
 to bypass the declaration check for non-static members.

Reviewed By: @int3

Differential Revision: D1664175

Signature: t1:1664175:1415309554:52fc0b92cf403d8a158d8eec62fe0c5a011752f1