Add a stack chain class to the stack trace package.

This class uses zones to track stack traces across asynchronous boundaries. It
will aid considerably in debugging errors in heavily-asynchronous programs.

R=floitsch@google.com, rnystrom@google.com
BUG=7040

Review URL: https://codereview.chromium.org//75863004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@30738 260f80e4-7a28-3924-810f-c04153c831b5