Add a couple functions to package:stack_trace. This adds a function to convert a Trace to a VM-style StackTrace object, and to parse a Trace-style string into a Trace. R=jmesserly@google.com Review URL: https://codereview.chromium.org//18029023 git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart/pkg/stack_trace@24611 260f80e4-7a28-3924-810f-c04153c831b5