Fix prefer_final_fields with generic super-classes (#1298)

Fix prefer_final_fields with generic super-classes

Author: srawlins

lib/src/util/dart_type_utilities.dart

@@ -7,6 +7,7 @@ import 'dart:collection';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
+import 'package:analyzer/src/dart/element/member.dart';
 
 typedef bool AstNodePredicate(AstNode node);
 
@@ -16,8 +17,25 @@ class DartTypeUtilities {
       (type is InterfaceType &&
           extendsClass(type.superclass, className, library));
 
-  static Element getCanonicalElement(Element element) =>
-      element is PropertyAccessorElement ? element.variable : element;
+  static Element getCanonicalElement(Element element) {
+    if (element is PropertyAccessorElement) {
+      final variable = element.variable;
+      if (variable is FieldMember) {
+        // A field element defined in a parameterized type where the values of
+        // the type parameters are known.
+        //
+        // This concept should be invisible when comparing FieldElements, but a
+        // bug in the analyzer causes FieldElements to not evaluate as
+        // equivalent to equivalent FieldMembers. See
+        // https://github.com/dart-lang/sdk/issues/35343.
+        return variable.baseElement;
+      } else {
+        return variable;
+      }
+    } else {
+      return element;
+    }
+  }
 
   static Element getCanonicalElementFromIdentifier(AstNode rawNode) {
     if (rawNode is Expression) {

test/rules/prefer_final_fields.dart

@@ -73,7 +73,7 @@ class E {
   }
 }
 
-class F{
+class F {
   var _array = new List<int>(5); // LINT
 
   void foo() {
@@ -85,9 +85,20 @@ class F{
 class IdBug686 {
   static int _id = 0;
   static String generateId({prefix: String}) {
-    return (prefix ?? "id") + "-" +
-        (_id++).toString() + "-" + _foo();
+    return (prefix ?? "id") + "-" + (_id++).toString() + "-" + _foo();
   }
 
   static String _foo() => '';
 }
+
+// analyzer's `FieldMember` vs `FieldElement` caused
+// https://github.com/dart-lang/sdk/issues/34417
+abstract class GenericBase<T> {
+  int _current = 0; // OK
+}
+
+class GenericSub extends GenericBase<int> {
+  void test() {
+    _current = 1;
+  }
+}