The test is indeed wrong, and it shouldn't pass.
Even if we allowed const Symbol("_x="), it would not create a symbol for the private setter name (or a name corresponding to any source entity). We don't allow it, the Symbol constructor docs say:

String must be either

a valid public Dart identifier (that is, an identifier not starting with _),

such an identifier followed by = (a setter name),

the name of a declarable operator (one of +, -, *, /, %, ~/, &, |, ^, ~, <<, >>, <, <=, >, >=, ==, [], []=, or unary-),

any of the above preceded by any number of qualifiers, where a qualifier is a non-private identifier followed by .,

or the empty string (the default name of a library with no library name declaration).

The "private identifier" _x= is not allowed, and the constructor should throw.

I'm guessing it "works" in the CFE because the CFE defers some constant evaluation to the back-ends, and at least the VM has never enforced input validation on the Symbol constructor (const constructors really can't check anything, so it requires compiler cooperation to check the constant invocations of Symbol, and the VM never implemented that).
The VM should still not create a symbol that is equal to the setter for the private _x member from a string, and I think it does, otherwise the equals check in the test would fail when the setter is used.
That is a bug (and a dastardly one because people might depend on it).

There is currently no sanctioned and easy way to create a symbol for a private setter name (you can capture it with noSuchMethod, or use dart:mirrors, but that's it). We should probably extend the syntax to allow #_x= as a symbol literal. No space before the = and it's tokenized as one symbol.

There would be a reason to allow Symbol("_foo") because _foo is a valid non-private library name, and there is no other way to create that symbol. We do not allow that yet.

mock_writable_final_private_field_test needs triage #34377

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