unused_element

The declaration '{0}' isn't referenced.

The analyzer produces this diagnostic when a private declaration isn't referenced in the library that contains the declaration. The following kinds of declarations are analyzed:

• Private top-level declarations and all of their members
• Private members of public declarations

Not all references to an element will mark it as "used":

• Assigning a value to a top-level variable (with a standard = assignment, or a null-aware ??= assignment) does not count as using it.
• Referring to an element in a doc comment reference does not count as using it.
• Referring to a class, mixin, or enum on the right side of an is expression does not count as using it.

Assuming that no code in the library references _C, the following code produces this diagnostic:

class _C {}

If the declaration isn't needed, then remove it.

If the declaration is intended to be used, then add the code to use it.