Restructure inlining of Bigint accessors, use consistent approach, fixes a potential crash.

R=regis@google.com

Review URL: https://codereview.chromium.org//736113002

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41840 260f80e4-7a28-3924-810f-c04153c831b5