[vm_snapshot_analysis] Extract logic from `summary` command so that it can be used by package consumers

[kenzieschmoll]

Similar to 4079c37, we'd like to be able to consume information from summary in Dart DevTools. In particular, we are interested in the dominator tree generated from the precompiler trace.

We are looking for something like this:
Map<String, dynamic> dominatorTreeFromTrace(Object precompilerTrace)
where the returned output is some json format for the dominator tree.

@mraleph

[kenzieschmoll]

addressed in 42dfb9d