Don't iterate over IC data that may be updated by the mutator thread.

runtime/vm/flow_graph_compiler.cc

 // Copyright (c) 2013, 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_XXX.
 
 #include "vm/flow_graph_compiler.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
@@ skipping 1176 matching lines @@
                                deopt_id, token_pos, locs);
     return;
   }
   if (FLAG_always_megamorphic_calls) {
     EmitMegamorphicInstanceCall(ic_data, argument_count,
                                 deopt_id, token_pos, locs,
                                 CatchClauseNode::kInvalidTryIndex);
     return;
   }
   ASSERT(!ic_data.IsNull());
-  if (is_optimizing() && (ic_data.NumberOfUsedChecks() == 0)) {
+  if (is_optimizing() && (ic_data_in.NumberOfUsedChecks() == 0)) {
     // Emit IC call that will count and thus may need reoptimization at
     // function entry.
     ASSERT(may_reoptimize() || flow_graph().IsCompiledForOsr());
     switch (ic_data.NumArgsTested()) {
       case 1:
         EmitOptimizedInstanceCall(
             *StubCode::OneArgOptimizedCheckInlineCache_entry(), ic_data,
             argument_count, deopt_id, token_pos, locs);
         return;
       case 2:
@@ skipping 782 matching lines @@
 
 
 void FlowGraphCompiler::FrameStateClear() {
   ASSERT(!is_optimizing());
   frame_state_.TruncateTo(0);
 }
 #endif  // defined(DEBUG) && !defined(TARGET_ARCH_DBC)
 
 
 }  // namespace dart