[element model] migrate unreachable_from_main

Bug: dart-lang/linter#5099
Change-Id: I31ee1aff4cbca63b8b66d6fe31a6e901d104fa71
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/394005
Reviewed-by: Konstantin Shcheglov <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Phil Quitslund <[email protected]>

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -8076,6 +8076,8 @@ final class ForStatementImpl extends StatementImpl
 /// A declaration of a fragment of an element.
 @experimental
 abstract final class FragmentDeclaration implements Declaration {
+  // TODO(pq): move `declaredFragment` into `Declaration` and remove this class.
+
   /// The fragment declared by this declaration.
   ///
   /// Returns `null` if the AST structure hasn't been resolved.

pkg/analyzer/lib/src/dart/element/element.dart

@@ -1970,6 +1970,9 @@ class DynamicElementImpl2 extends TypeDefiningElementImpl2 {
   @override
   Null get documentationComment => null;
 
+  @override
+  Element2? get enclosingElement2 => null;
+
   @override
   DynamicElementImpl get firstFragment => DynamicElementImpl.instance;
 
@@ -8431,6 +8434,9 @@ class NeverElementImpl extends ElementImpl
   @override
   NeverElementImpl2 get element => NeverElementImpl2.instance;
 
+  @override
+  Element2? get enclosingElement2 => null;
+
   @override
   Null get enclosingFragment => null;
 
@@ -8480,6 +8486,9 @@ class NeverElementImpl2 extends TypeDefiningElementImpl2 {
   @override
   Null get documentationComment => null;
 
+  @override
+  Element2? get enclosingElement2 => null;
+
   @override
   NeverElementImpl get firstFragment => NeverElementImpl.instance;
 

pkg/analyzer/lib/src/dart/element/inheritance_manager3.dart

@@ -401,9 +401,7 @@ class InheritanceManager3 {
     if (elements == null) {
       return null;
     }
-    return elements
-        .map((fragment) => (fragment as ExecutableFragment).element)
-        .toList();
+    return elements.map((fragment) => fragment.asExecutableElement2).toList();
   }
 
   /// Return all members of mixins, superclasses, and interfaces that a member