Revert r28050 due to crashes with --optimization-counter-threshold=5.

runtime/vm/flow_graph_optimizer.h

 // 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.
 
 #ifndef VM_FLOW_GRAPH_OPTIMIZER_H_
 #define VM_FLOW_GRAPH_OPTIMIZER_H_
 
 #include "vm/intermediate_language.h"
 #include "vm/flow_graph.h"
 
@@ skipping 230 matching lines @@
 
 // Sparse conditional constant propagation and unreachable code elimination.
 // Assumes that use lists are computed and preserves them.
 class ConstantPropagator : public FlowGraphVisitor {
  public:
   ConstantPropagator(FlowGraph* graph,
                      const GrowableArray<BlockEntryInstr*>& ignored);
 
   static void Optimize(FlowGraph* graph);
 
-  // (1) Visit branches to optimize away unreachable blocks discovered  by range
-  // analysis.
-  // (2) Eliminate branches that have the same true- and false-target: For
-  // example, this occurs after expressions like
-  //
-  // if (a == null || b == null) {
-  //   ...
-  // }
-  //
-  // where b is known to be null.
+  // Only visit branches to optimize away unreachable blocks discovered
+  // by range analysis.
   static void OptimizeBranches(FlowGraph* graph);
 
   // Used to initialize the abstract value of definitions.
   static RawObject* Unknown() { return Object::unknown_constant().raw(); }
 
  private:
   void Analyze();
   void VisitBranches();
   void Transform();
-  void EliminateRedundantBranches();
 
   void SetReachable(BlockEntryInstr* block);
   void SetValue(Definition* definition, const Object& value);
 
   // Assign the join (least upper bound) of a pair of abstract values to the
   // first one.
   void Join(Object* left, const Object& right);
 
   bool IsUnknown(const Object& value) {
     return value.raw() == unknown_.raw();
@@ skipping 99 matching lines @@
 // Optimize spill stores inside try-blocks by identifying values that always
 // contain a single known constant at catch block entry.
 class TryCatchAnalyzer : public AllStatic {
  public:
   static void Optimize(FlowGraph* flow_graph);
 };
 
 }  // namespace dart
 
 #endif  // VM_FLOW_GRAPH_OPTIMIZER_H_