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We introduced the SKIP_SETUP sentinel value when converting Field and its subclasses to ES6 class syntax, because super must be called before any other code in a subclass constructor, breaking the previous mechanism where subclasses would set some properties before calling their superclass constructor. SKIP_SETUP was a singleton value of class Sentinel. Recently, in PR #6639 @btw17 introduced the isSentinel type predicate to improve the typing of Field. Unfortunately, there were some aspects of this change that were not very elegant: - isSentinel was declared as a static method on Field (rather than on Sentinel itself). - It only checks against the specific value SKIP_SETUP, rather than checking if the argument is instanceof Sentinel (though perhaps this is for efficiency?) Additionally - as a result of the migration from ES6 to TS, and predating PR #6639 - The signature for the Field constructor's first argument was typed T|Sentinel, with subclass constructors generally being <some type(s)>|Sentinel. This creates a small problem when attempting to port Fields from core to plugins, because Sentinel is not reexported by core/utils.ts (and therefore not from core/blockly.ts either). The latter problem could be solved simply by reexporting Sentinel, or by having plugin constructors not accept SKIP_SETUP (though this potentially makes them more difficult to subclass), but neither is particularly desirable. Instead, this PR proposes that we: - Make Field.SKIP_SETUP a (unique) Symbol. - Change the value argument to the Field constructor to accept T|typeof Field.SKIP_SETUP - where typeof Field.SKIP_SETUP is (like a literal type) a type that accepts just the single value SKIP_SETUP. - Remove the Sentinel class and core/utils/sentinel.ts. Not treating this as a breaking change: - Removes Field.isSentinel - though this addition has not yet been published, so it can only break our own as-yet-unreleased code in samples. - Changes the type of Field.SKIP_SETUP and the first argument of the Field constructor from Sentinel to typeof SKIP_SETUP (a unique Symbol) - but given that Sentinel has never been exported this should not break any actual external code.