Show a dominator tree in the memory page to display which objects are holding on to each other

Some helpful links for dominator trees, all related to the Devtools app size tool:
- PR to add dominator tree and call graph to app size page: https://github.com/flutter/devtools/pull/2300
- SDK change to add generateCallGraphWithDominators method to package:vm_snapshot_analysis https://dart-review.googlesource.com/c/sdk/+/160342
- PR to add dominator tree and call graph for app size diffs: https://github.com/flutter/devtools/pull/2344
- public docs on using dominator tree and call graph in app size page: https://flutter.dev/docs/development/tools/devtools/app-size#dominator-tree-and-call-graph

Start with the linked DevTools pull requests for dominator trees and call graphs, then review the vm_snapshot_analysis SDK change adding generateCallGraphWithDominators. Trace how that data is presented in the app-size page and adapt the approach for the memory page; done means the memory page displays which objects retain others as a dominator tree.

Written by the indexing model from the issue text.