Use SAR for positive divident by a power-of two constant divisor.

runtime/vm/flow_graph_optimizer.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/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/flow_graph_builder.h"
 #include "vm/flow_graph_compiler.h"
@@ skipping 507 matching lines @@
   Function& target = Function::Handle();
   const intptr_t len = ic_data.NumberOfChecks();
   for (intptr_t i = 0; i < len; i++) {
     ic_data.GetCheckAt(i, &class_ids, &target);
     if (class_ids[arg_n] != kSmiCid) return false;
   }
   return true;
 }
 
 
-// Returns array classid to load from, array and idnex value
+// Returns array classid to load from, array and index value
 
 intptr_t FlowGraphOptimizer::PrepareIndexedOp(InstanceCallInstr* call,
                                               intptr_t class_id,
                                               Value** array,
                                               Value** index) {
   *array = call->ArgumentAt(0)->value();
   *index = call->ArgumentAt(1)->value();
   // Insert class check and index smi checks and attach a copy of the
   // original environment because the operation can still deoptimize.
   AddCheckClass(call, (*array)->Copy());
@@ skipping 4022 matching lines @@
 
   if (FLAG_trace_constant_propagation) {
     OS::Print("\n==== After constant propagation ====\n");
     FlowGraphPrinter printer(*graph_);
     printer.PrintBlocks();
   }
 }
 
 
 }  // namespace dart