fix #27135 and fix #27449 strong mode does not use propagated types

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 analyzer.src.generated.element_resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/syntactic_entity.dart';
@@ skipping 1829 matching lines @@
   /**
    * Determines if the [propagatedType] of the invoke is better (more specific)
    * than the [staticType]. If so it will be returned, otherwise returns null.
    */
   // TODO(jmesserly): can we refactor Resolver.recordPropagatedTypeIfBetter to
   // get some code sharing? Right now, this method is to support
   // `staticInvokeType` and `propagatedInvokeType`, and the one in Resolver is
   // for `staticType` and `propagatedType` on Expression.
   DartType _propagatedInvokeTypeIfBetter(
       DartType propagatedType, DartType staticType) {
-    if (propagatedType != null &&
-        (staticType == null || propagatedType.isMoreSpecificThan(staticType))) {
-      return propagatedType;
-    } else {
+    if (_resolver.strongMode || propagatedType == null) {
       return null;
     }
+    if (staticType == null || propagatedType.isMoreSpecificThan(staticType)) {
+      return propagatedType;
+    }
+    return null;
   }
 
   /**
    * Record that the given [node] is undefined, causing an error to be reported
    * if appropriate. The [declaringElement] is the element inside which no
    * declaration was found. If this element is a proxy, no error will be
    * reported. If null, then an error will always be reported. The [errorCode]
    * is the error code to report. The [arguments] are the arguments to the error
    * message.
    */
@@ skipping 772 matching lines @@
 
   @override
   Element get staticElement => null;
 
   @override
   dynamic/*=E*/ accept/*<E>*/(AstVisitor/*<E>*/ visitor) => null;
 
   @override
   void visitChildren(AstVisitor visitor) {}
 }