Update final fields inference according to the clarified specification.

pkg/analyzer/lib/src/summary/link.dart

 // Copyright (c) 2016, 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.
 
 /**
  * This library is capable of producing linked summaries from unlinked
  * ones (or prelinked ones).  It functions by building a miniature
  * element model to represent the contents of the summaries, and then
  * scanning the element model to gather linked information and adding
  * it to the summary data structures.
@@ skipping 4679 matching lines @@
  * linking.
  */
 class SyntheticVariableElementForLink implements PropertyInducingElementImpl {
   PropertyAccessorElementForLink_Executable _getter;
   PropertyAccessorElementForLink_Executable _setter;
 
   @override
   PropertyAccessorElementForLink_Executable get getter => _getter;
 
   @override
+  bool get isFinal => _setter == null;
+
+  @override
   bool get isSynthetic => true;
 
   @override
   PropertyAccessorElementForLink_Executable get setter => _setter;
 
   @override
   void set type(DartType inferredType) {}
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
@@ skipping 567 matching lines @@
 }
 
 /**
  * This exception is thrown when [ExprTypeComputer] cannot inference the type.
  */
 class _InferenceFailedError {
   final String message;
 
   _InferenceFailedError(this.message);
 }