Problems with null check in NonParameterVariableElementImpl.enclosingElement3

[stereotype441]

During my development of https://dart-review.googlesource.com/c/sdk/+/400660, I ran into a subtle bug where the following null check (in the class NonParameterVariableElementImpl) was failing:

  @override
  Element get enclosingElement3 => super.enclosingElement3!;

After discussion with @bwilkerson, I suspect this null check doesn't belong here. Variable elements can have a null enclosing element, if the variable is in a local function, as you can see from the declaration of _NonTopLevelVariableOrParameter.enclosingElement2:

  @override
  Element2? get enclosingElement2 {
    // TODO(dantup): Can we simplify this code and inline it into each class?

    var enclosingFunction = _enclosingFunction;
    return switch (enclosingFunction) {
      // There is no enclosingElement for a local function so we need to
      // determine whether our enclosing FunctionElementImpl is a local function
      // or not.
      // TODO(dantup): Is the real issue here that we're getting
      //  FunctionElementImpl here that should be LocalFunctionElementImpl?
      FunctionElementImpl()
          when enclosingFunction.enclosingElement3 is ExecutableElementImpl ||
              enclosingFunction.enclosingElement3 is VariableElementImpl =>
        null,
      // GenericFunctionTypeElementImpl currently implements Fragment but throws
      // if we try to access `element`.
      GenericFunctionTypeElementImpl() => null,
      // Otherwise, we have a valid enclosing element.
      Fragment(:var element) => element,
      _ => null,
    };
  }

This leads to some fragility, since local functions are fairly rare. I wonder if perhaps we should standardize on saying that either that (a) all local variables have a null enclosingElement2, or (b) all local variables have a non-null enclosingElement2.

To repro the failure, check out patchset 3 of https://dart-review.googlesource.com/c/sdk/+/400660 and change lines 1973-1974 of pkg/analyzer/lib/src/dart/resolver/resolution_visitor.dart from this:

          ..enclosingElement3 =
              first.firstFragment.enclosingFragment!.element.asElement!

to this:

          ..enclosingElement3 = first.enclosingElement2.asElement

Then run the test suite pkg/analyzer/test/src/dart/resolution/switch_expression_test.dart.

[bwilkerson]

I'll note that LabelElement2 has the same issue (using ! when the enclosing element can be null).

To make things worse, LocalFunctionElement always returns null, which is also wrong but in a different way.