Implement ScopedThreadLocal as a mechanism to keep dart user state associated with isolate group callbacks

[aam]

This issue tracks implementation of ScopedThreadLocal as proposed in dart-lang/language#4515.
It will be used to replace use of static fields in dart code for various internal data structures like

sdk/sdk/lib/internal/internal.dart

Line 1127 in 9a86ae5

final List<Object> toStringVisiting = [];

[lrhn]

With this, we might even be able to do Zone.current, but probably still not the microtask queue, so dart:async won't be viable anyway.