[vm/concurrency] Make static-const fields that are purely used by runtime operate on initial field table

For normal Dart code a usage of a constant (e.g. from a static const
field) is "inlined": It is represented as a `ConstantExpression` AST
node with a reference to the "evaluated" `Constant` node.
=> Static const fields are never accessed by generated code.

We would therefore not need to represent static const fields in
Kernel/VM anymore. Though there are a few exceptions where the VM's
runtime code needs access to them, which is why they are kept.

The VM runtime code doesn't need an isolate-specific slot for such
static const fields (it's a waste).

=> This CL changes the runtime code that accesses such fields to operate
   on the `IsolateGroup`s initial field table instead.

Issue https://github.com/dart-lang/sdk/issues/36097

TEST=Pure refactoring - relying on existing test coverage.

Change-Id: I5994811a1c1016b2f523b318b698c9e052a98c29
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/177709
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>