[vm] Remove Dart_MakeExternalString and --support-externalizable-strings

runtime/vm/redundancy_elimination.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.
 
 #if !defined(DART_PRECOMPILED_RUNTIME)
 
 #include "vm/redundancy_elimination.h"
 
 #include "vm/bit_vector.h"
 #include "vm/flow_graph.h"
 #include "vm/hash_map.h"
 #include "vm/il_printer.h"
 #include "vm/intermediate_language.h"
 #include "vm/stack_frame.h"
 
 namespace dart {
 
 DEFINE_FLAG(bool, dead_store_elimination, true, "Eliminate dead stores");
 DEFINE_FLAG(bool, load_cse, true, "Use redundant load elimination.");
 DEFINE_FLAG(bool,
             trace_load_optimization,
             false,
             "Print live sets for load optimization pass.");
 
 // Quick access to the current zone.
 #define Z (zone())
 
 class CSEInstructionMap : public ValueObject {
  public:
-  // Right now CSE and LICM track a single effect: possible externalization of
-  // strings.
+  // Right now CSE and LICM track a single effect which is no longer used.
+  // TODO(alexmarkov): cleanup.

[Vyacheslav Egorov (Google), 2017/08/18 05:50:46]

I suggest using 

TODO(dartbug.com/30474) 

instead of 

TODO(alexmarkov)

Then it's easier to find outstanding tasks. 

[alexmarkov, 2017/08/21 21:17:07]

Done (in the next CL).

   // Other effects like modifications of fields are tracked in a separate load
   // forwarding pass via Alias structure.
   COMPILE_ASSERT(EffectSet::kLastEffect == 1);
 
   CSEInstructionMap() : independent_(), dependent_() {}
   explicit CSEInstructionMap(const CSEInstructionMap& other)
       : ValueObject(),
         independent_(other.independent_),
         dependent_(other.dependent_) {}
 
@@ skipping 3289 matching lines @@
       } else {
         join->phis_->TruncateTo(to_index);
       }
     }
   }
 }
 
 }  // namespace dart
 
 #endif  // !defined(DART_PRECOMPILED_RUNTIME)