Experimental code to detect transitive closure of parser code which causes new-space allocations

Experimental code to detect transitive closure of parser code which causes new-space allocations

This CL adds a `Thread::inside_parser_` field which is used for keeping
track of whether upper frames are inside the parser or not.

The field is kept up-to-date via `EnterParserScope`/`LeaveParserScope`
and is checked in Dart heap allocation code to ensure we never allocate
in Heap::kNew from parser (or transitively called code) -- (the
Parser::allocation_space_ has been fixed to Heap::kOld for this purpose).

-> This code allows us to see all Heap::kNew allocation sites
   transitively reachable by the parser.

(not to be committed)