[flutter_tools] update to vm_service 5.2.0, update to dwds 7.0.0 (#69067)

Rolls in several fixes to web tooling, including better handling of absolute file imports. Updates to latest vm service to unblock null safety mode query

Co-authored-by: Gary Roumanis <grouma@google.com>
Co-authored-by: Anna Gringauze <annagrin@google.com>