Add PrefixFragment without implementing it

This is to make sure that we're in agreement about the changes we
discussed yesterday and to capture them before I forgot.

Change-Id: I629bc97122c688f70af701b1c6c6e32801a77311
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/379222
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/dart/element/element2.dart

@@ -54,7 +54,6 @@ import 'package:analyzer/dart/element/element.dart'
         ElementAnnotation,
         ElementKind,
         ElementLocation,
-        ImportElementPrefix,
         LibraryLanguageVersion,
         NamespaceCombinator;
 import 'package:analyzer/dart/element/nullability_suffix.dart';
@@ -839,10 +838,10 @@ abstract class LibraryImport {
   /// The [Namespace] that this directive contributes to the containing library.
   Namespace get namespace;
 
-  /// The prefix that was specified as part of the import directive.
+  /// The prefix fragment that was specified as part of the import directive.
   ///
   /// Returns `null` if there was no prefix specified.
-  ImportElementPrefix? get prefix;
+  PrefixFragment? get prefix2;
 
   /// The interpretation of the URI specified in the directive.
   DirectiveUri get uri;
@@ -919,11 +918,13 @@ abstract class PatternVariableElement2 implements LocalVariableElement2 {
 /// A prefix used to import one or more libraries into another library.
 ///
 /// Clients may not extend, implement or mix-in this class.
-abstract class PrefixElement2 implements Element2 {
-  /// The library that encloses this element.
+abstract class PrefixElement2 implements Element2, _Fragmented {
   @override
   LibraryElement2 get enclosingElement2;
 
+  @override
+  PrefixFragment get firstFragment;
+
   /// The imports that share this prefix.
   List<LibraryImport> get imports2;
 
@@ -938,6 +939,23 @@ abstract class PrefixElement2 implements Element2 {
   Scope get scope;
 }
 
+/// The portion of a [PrefixElement2] contributed by a single declaration.
+///
+/// Clients may not extend, implement or mix-in this class.
+abstract class PrefixFragment implements Fragment {
+  @override
+  PrefixElement2 get element;
+
+  @override
+  LibraryFragment? get enclosingFragment;
+
+  @override
+  PrefixFragment? get nextFragment;
+
+  @override
+  PrefixFragment? get previousFragment;
+}
+
 abstract class PromotableElement2 implements VariableElement2 {}
 
 abstract class PromotableFragment implements VariableFragment {}

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

@@ -5272,6 +5272,10 @@ class LibraryImportElementImpl extends _ExistingElementImpl
     return Namespace.EMPTY;
   }
 
+  @override
+  PrefixFragment? get prefix2 =>
+      throw UnimplementedError('Prefix fragments are not yet supported');
+
   @override
   bool operator ==(Object other) {
     return identical(this, other);
@@ -6983,6 +6987,10 @@ class PrefixElementImpl extends _ExistingElementImpl
     return _enclosingElement3 as CompilationUnitElementImpl;
   }
 
+  @override
+  PrefixFragment get firstFragment =>
+      throw UnimplementedError('Prefix fragments are not yet supported');
+
   @override
   List<LibraryImportElementImpl> get imports {
     return enclosingElement.libraryImports