VM: [Kernel] Ensure that result of v.operator[]=(...) invocations is never used.

runtime/vm/kernel_to_il.cc

 // 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.
 
 #include <map>
 #include <set>
 #include <string>
 
 #include "vm/kernel_to_il.h"
 
@@ skipping 4863 matching lines @@
   // If we have a special operation (e.g. +/-/==) we mark both arguments as
   // to be checked.
   if (token_kind != Token::kILLEGAL) {
     ASSERT(argument_count <= 2);
     num_args_checked = argument_count;
   }
 
   fragment_ = instructions + InstanceCall(node->position(), name, token_kind,
                                           argument_count, argument_names,
                                           num_args_checked);
+  // Later optimization passes assume that result of a x.[]=(...) call is not
+  // used. We must guarantee this invariant because violation will lead to an
+  // illegal IL once we replace x.[]=(...) with a sequence that does not
+  // actually produce any value. See http://dartbug.com/29135 for more details.
+  if (name.raw() == Symbols::AssignIndexToken().raw()) {
+    fragment_ += Drop();
+    fragment_ += NullConstant();
+  }
 }
 
 
 void FlowGraphBuilder::VisitDirectMethodInvocation(
     DirectMethodInvocation* node) {
   const dart::String& method_name = H.DartProcedureName(node->target());
   const Function& target = Function::ZoneHandle(
       Z, LookupMethodByMember(node->target(), method_name));
 
   intptr_t argument_count = node->arguments()->count() + 1;
@@ skipping 1549 matching lines @@
     thread->clear_sticky_error();
     return error.raw();
   }
 }
 
 
 }  // namespace kernel
 }  // namespace dart
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)