Change package:vm_service BoundVariable.value from "dynamic" to "Response"

[DanTup]

Noted at #51916 (comment). The current type of BoundVariable.value is dynamic:

sdk/pkg/vm_service/lib/src/vm_service.dart

Lines 3088 to 3089 in 57ba99c

	/// [value] can be one of [InstanceRef], [TypeArgumentsRef] or [Sentinel].
	dynamic value;

All of the types it can be implement/extend Response so this could be Response.

The problem with dynamic is that it's easy to pass to any function and get a runtime error (which is what #51916 was, assuming it was always InstanceRef).

@bkonyi notes this code is generated so might not be trivial to know the types all share a common base.

[bkonyi]

FYI @derekxu16. This isn't high priority, but we'll probably want to do a breaking release at some point to make this change. It'd probably be sufficient to change all of our instances of dynamic in vm_service classes to Object to avoid having to figure out the common base class of each type in a union.

[DanTup]

It'd probably be sufficient to change all of our instances of dynamic in vm_service classes to Object

Yeah, that sounds like a much simpler change and would've also prevented the issue I hit :-)