It is the setter that can have a result, but not the StoreInstanceFieldNode.

runtime/vm/code_generator_x64.cc

 // Copyright (c) 2012, 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.
 
 #include "vm/globals.h"  // Needed here to get TARGET_ARCH_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/code_generator.h"
 
 #include "lib/error.h"
@@ skipping 943 matching lines @@
   MarkDeoptPoint(node->id(), node->token_index());
   __ popq(RAX);  // Value.
   if (FLAG_enable_type_checks) {
     GenerateAssertAssignable(node->id(),
                              node->value()->token_index(),
                              AbstractType::ZoneHandle(node->field().type()),
                              String::ZoneHandle(node->field().name()));
   }
   __ popq(R10);  // Instance.
   __ StoreIntoObject(R10, FieldAddress(R10, node->field().Offset()), RAX);
-  if (IsResultNeeded(node)) {
-    // The result is the input value.
-    __ pushq(RAX);
-  }
+  ASSERT(!IsResultNeeded(node));
 }
 
 
 // Expects array and index on stack and returns result in RAX.
 void CodeGenerator::GenerateLoadIndexed(intptr_t node_id,
                                         intptr_t token_index) {
   // Invoke the [] operator on the receiver object with the index as argument.
   const String& operator_name =
       String::ZoneHandle(String::NewSymbol(Token::Str(Token::kINDEX)));
   const int kNumArguments = 2;  // Receiver and index.
@@ skipping 1729 matching lines @@
   const Error& error = Error::Handle(
       Parser::FormatError(script, token_index, "Error", format, args));
   va_end(args);
   Isolate::Current()->long_jump_base()->Jump(1, error);
   UNREACHABLE();
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64