Revert "Allow bringForward to "initial run""

This reverts commit 29976d7922c5c3e99f0469e3db11b391d39a2b24.
Fixing stillValid code

We need to take into account that symbols can can have non-class owners yet still be valid.
Previously such symbols were treated as invalid. Now they are valid if their owners are valid.

The commit supersedes

29976d7922c5c3e99f0469e3db1

which got reverted.
Added a unique identifier to trees.

Used for debugging purposes for now. Might be used for replaying IDE interactions later.
Some more debug info.