Reapply r18377 it was reverted due to the unrelated bug it surfaced.

runtime/vm/flow_graph_compiler_x64.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_X64.
 #if defined(TARGET_ARCH_X64)
 
 #include "vm/flow_graph_compiler.h"
 
 #include "lib/error.h"
 #include "vm/ast_printer.h"
 #include "vm/dart_entry.h"
 #include "vm/il_printer.h"
 #include "vm/locations.h"
 #include "vm/object_store.h"
 #include "vm/parser.h"
 #include "vm/stub_code.h"
 #include "vm/symbols.h"
 
 namespace dart {
 
 DEFINE_FLAG(bool, trap_on_deoptimization, false, "Trap on deoptimization.");
 DECLARE_FLAG(int, optimization_counter_threshold);
 DECLARE_FLAG(bool, print_ast);
 DECLARE_FLAG(bool, print_scopes);
-DECLARE_FLAG(bool, use_sse41);
+DECLARE_FLAG(bool, eliminate_type_checks);
 
 
 FlowGraphCompiler::~FlowGraphCompiler() {
   // BlockInfos are zone-allocated, so their destructors are not called.
   // Verify the labels explicitly here.
   for (int i = 0; i < block_info_.length(); ++i) {
     ASSERT(!block_info_[i]->label.IsLinked());
     ASSERT(!block_info_[i]->label.HasNear());
   }
 }
@@ skipping 517 matching lines @@
          (!dst_type.IsDynamicType() && !dst_type.IsObjectType()));
   __ pushq(RCX);  // Store instantiator.
   __ pushq(RDX);  // Store instantiator type arguments.
   // A null object is always assignable and is returned as result.
   const Immediate& raw_null =
       Immediate(reinterpret_cast<intptr_t>(Object::null()));
   Label is_assignable, runtime_call;
   __ cmpq(RAX, raw_null);
   __ j(EQUAL, &is_assignable);
 
+  if (!FLAG_eliminate_type_checks) {
+    // If type checks are not eliminated during the graph building then
+    // a transition sentinel can be seen here.
+    __ CompareObject(RAX, Object::transition_sentinel());
+    __ j(EQUAL, &is_assignable);
+  }
+
   // Generate throw new TypeError() if the type is malformed.
   if (dst_type.IsMalformed()) {
     const Error& error = Error::Handle(dst_type.malformed_error());
     const String& error_message = String::ZoneHandle(
         Symbols::New(error.ToErrorCString()));
     __ PushObject(Object::ZoneHandle());  // Make room for the result.
     __ pushq(RAX);  // Push the source object.
     __ PushObject(dst_name);  // Push the name of the destination.
     __ PushObject(error_message);
     GenerateCallRuntime(token_pos,
@@ skipping 1126 matching lines @@
 void ParallelMoveResolver::Exchange(const Address& mem1, const Address& mem2) {
   __ Exchange(mem1, mem2);
 }
 
 
 #undef __
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64