Fix type inference of PropertySet when the receiver is a generic class.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.dart

 // Copyright (c) 2017, 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 file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 1427 matching lines @@
         } else {
           // References to fields and setters can't be relied upon for top level
           // inference.
           inferrer.recordNotImmediatelyEvident(fileOffset);
         }
       }
     }
     // To replicate analyzer behavior, we base type inference on the write
     // member.  TODO(paulberry): would it be better to use the read member when
     // doing compound assignment?
-    var writeContext = writeMember?.setterType;
+    var writeContext = inferrer.getSetterType(writeMember, receiverType);
     var inferredType = _inferRhs(inferrer, writeContext);
     inferrer.listener.propertyAssignExit(desugared, inferredType);
     return inferredType;
   }
 }
 
 /// Shadow object for [PropertyGet].
 class KernelPropertyGet extends PropertyGet implements KernelExpression {
   KernelPropertyGet(Expression receiver, Name name, [Member interfaceTarget])
       : super(receiver, name, interfaceTarget);
@@ skipping 757 matching lines @@
   }
 
   transformChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 
   visitChildren(v) {
     return internalError("Internal error: Unsupported operation.");
   }
 }