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

runtime/vm/redundancy_elimination.cc

Index: runtime/vm/redundancy_elimination.cc
diff --git a/runtime/vm/redundancy_elimination.cc b/runtime/vm/redundancy_elimination.cc
index 39d97e1e2adbc0b5e8652c3baeafe91da6f0c755..73eaecd0879004f42ba38a9f9489a8fad815a1c6 100644
--- a/runtime/vm/redundancy_elimination.cc
+++ b/runtime/vm/redundancy_elimination.cc
@@ -27,8 +27,8 @@ DEFINE_FLAG(bool,
 
 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);