[analyzer] Support enclosingFragments that are local variables

Fixes at least some of Dart-Code/Dart-Code#5369.

Change-Id: I367969335573e4659fc0932a5751d547b0942836
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/401100
Reviewed-by: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: DanTup

pkg/analysis_server/test/lsp/document_highlights_test.dart

@@ -26,6 +26,44 @@ void f() {
   var _ = 2;
   print(/*[1*/_/*1]*/);
 }
+''');
+
+  Future<void> test_dartCode_issue5369_field() => _testMarkedContent('''
+class A {
+  var /*[0*/^a/*0]*/ = [''].where((_) => true).toList();
+  List<String> f() {
+    return /*[1*/a/*1]*/;
+  }
+}
+
+var a; // Not a reference
+''');
+
+  Future<void> test_dartCode_issue5369_functionType() => _testMarkedContent('''
+class A {
+  String m({ required String Function(String input) /*[0*/^f/*0]*/ }) {
+    return /*[1*/f/*1]*/('');
+  }
+}
+
+var f; // Not a reference
+''');
+
+  Future<void> test_dartCode_issue5369_localVariable() => _testMarkedContent('''
+class A {
+  List<String> f() {
+    var /*[0*/^a/*0]*/ = [''].where((_) => true).toList();
+    return /*[1*/a/*1]*/;
+  }
+}
+
+var a; // Not a reference
+''');
+
+  Future<void> test_dartCode_issue5369_topLevelVariable() =>
+      _testMarkedContent('''
+var /*[0*/^a/*0]*/ = [''].where((_) => true).toList();
+var b = /*[1*/a/*1]*/;
 ''');
 
   Future<void> test_forInLoop() => _testMarkedContent('''

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

@@ -5085,6 +5085,12 @@ class FunctionElementImpl extends ExecutableElementImpl
         return libraryFragment;
       case ExecutableFragment executableFragment:
         return executableFragment;
+      case LocalVariableFragment variableFragment:
+        return variableFragment;
+      case TopLevelVariableFragment variableFragment:
+        return variableFragment;
+      case FieldFragment fieldFragment:
+        return fieldFragment;
     }
     // Local functions cannot be augmented.
     throw UnsupportedError('This is not a fragment');