Enhance kernel AST verifier to detect dangling references

[alexmarkov]

Kernel AST verifier (pkg/kernel/lib/verifier.dart) can collect/mark all AST nodes which are directly reachable from Component and then check that all references within AST point to the reachable nodes. This would ensure there are no dangling references in the AST and would allow us to catch various bugs related to the incorrect tree-shaking earlier (currently such bugs show up only after kernel is serialized and then deserialized).