Update final fields inference according to the clarified specification.

pkg/analyzer/lib/src/task/strong_mode.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.task.strong_mode;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 96 matching lines @@
 
   /**
    * Return `true` if the list of [elements] contains only methods.
    */
   bool _allSameElementKind(
       ExecutableElement element, List<ExecutableElement> elements) {
     return elements.every((e) => e.kind == element.kind);
   }
 
   /**
-   * Compute the inferred type for the given property accessor [element]. The
-   * returned value is never `null`, but might be an error, and/or have the
-   * `null` type.
+   * Compute the inferred type for the given property [accessor]. The returned
+   * value is never `null`, but might be an error, and/or have the `null` type.
    */
   _FieldOverrideInferenceResult _computeFieldOverrideType(
-      ExecutableElement element) {
-    String name = element.displayName;
+      PropertyAccessorElement accessor) {
+    String name = accessor.displayName;
 
     var overriddenElements = <ExecutableElement>[];
     overriddenElements.addAll(
-        inheritanceManager.lookupOverrides(element.enclosingElement, name));
-    overriddenElements.addAll(
-        inheritanceManager.lookupOverrides(element.enclosingElement, '$name='));
+        inheritanceManager.lookupOverrides(accessor.enclosingElement, name));
+    if (overriddenElements.isEmpty || !accessor.variable.isFinal) {
+      List<ExecutableElement> overriddenSetters = inheritanceManager
+          .lookupOverrides(accessor.enclosingElement, '$name=');
+      overriddenElements.addAll(overriddenSetters);
+    }
 
     bool isCovariant = false;
     DartType impliedType;
     for (ExecutableElement overriddenElement in overriddenElements) {
       FunctionType overriddenType =
-          _toOverriddenFunctionType(element, overriddenElement);
+          _toOverriddenFunctionType(accessor, overriddenElement);
       if (overriddenType == null) {
         return new _FieldOverrideInferenceResult(false, null, true);
       }
 
       DartType type;
       if (overriddenElement.kind == ElementKind.GETTER) {
         type = overriddenType.returnType;
       } else if (overriddenElement.kind == ElementKind.SETTER) {
         if (overriddenType.parameters.length == 1) {
           ParameterElement parameter = overriddenType.parameters[0];
@@ skipping 464 matching lines @@
       AstNode parent = node.parent;
       if (parent is PropertyAccess && parent.propertyName == node ||
           parent is PrefixedIdentifier && parent.identifier == node) {
         isValid = false;
       }
     } else if (element is PropertyAccessorElement && !element.isStatic) {
       isValid = false;
     }
   }
 }