Fix propagated argument element bug. This fixes #25040.

pkg/analyzer/lib/src/generated/element_resolver.dart

 // Copyright (c) 2014, 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 engine.resolver.element_resolver;
 
 import 'dart:collection';
 
 import 'ast.dart';
 import 'element.dart';
@@ skipping 604 matching lines @@
       propagatedType = _getPropagatedType(target);
       //
       // If this method invocation is of the form 'C.m' where 'C' is a class,
       // then we don't call resolveInvokedElement(...) which walks up the class
       // hierarchy, instead we just look for the member in the type only.  This
       // does not apply to conditional method invocation (i.e. 'C?.m(...)').
       //
       bool isConditional = node.operator.type == sc.TokenType.QUESTION_PERIOD;
       ClassElementImpl typeReference = getTypeReference(target);
       if (typeReference != null) {
-        staticElement =
-            propagatedElement = _resolveElement(typeReference, methodName);
+        staticElement = _resolveElement(typeReference, methodName);
       } else {
         staticElement = _resolveInvokedElementWithTarget(
             target, staticType, methodName, isConditional);
-        propagatedElement = _resolveInvokedElementWithTarget(
-            target, propagatedType, methodName, isConditional);
+        // If we have propagated type information use it (since it should
+        // not be redundant with the staticType).  Otherwise, don't produce
+        // a propagatedElement which duplicates the staticElement.
+        if (propagatedType is InterfaceType) {
+          propagatedElement = _resolveInvokedElementWithTarget(
+              target, propagatedType, methodName, isConditional);
+        }
       }
     }
     staticElement = _convertSetterToGetter(staticElement);
     propagatedElement = _convertSetterToGetter(propagatedElement);
     //
     // Record the results.
     //
     methodName.staticElement = staticElement;
     methodName.propagatedElement = propagatedElement;
     ArgumentList argumentList = node.argumentList;
@@ skipping 1949 matching lines @@
 
   @override
   Element get staticElement => null;
 
   @override
   accept(AstVisitor visitor) => null;
 
   @override
   void visitChildren(AstVisitor visitor) {}
 }