[es6] Enable tail call optimization on clusterfuzz.

src/flag-definitions.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This file defines all of the flags.  It is separated into different section,
 // for Debug, Release, Logging and Profiling, etc.  To add a new flag, find the
 // correct section, and use one of the DEFINE_ macros, without a trailing ';'.
 //
 // This include does not have a guard, because it is a template-style include,
 // which can be included multiple times in different modes.  It expects to have
@@ skipping 183 matching lines @@
 // ES2015 const semantics are shipped
 DEFINE_NEG_VALUE_IMPLICATION(harmony_shipping, legacy_const, true)
 
 DEFINE_BOOL(promise_extra, true, "additional V8 Promise functions")
 // Removing extra Promise functions is staged
 DEFINE_NEG_IMPLICATION(harmony, promise_extra)
 
 // Activate on ClusterFuzz.
 DEFINE_IMPLICATION(es_staging, harmony_regexp_lookbehind)
 DEFINE_IMPLICATION(es_staging, move_object_start)
+DEFINE_IMPLICATION(es_staging, harmony_tailcalls)
 
 // Features that are still work in progress (behind individual flags).
 #define HARMONY_INPROGRESS(V)                                                \
   V(harmony_modules, "harmony modules")                                      \
   V(harmony_function_sent, "harmony function.sent")                          \
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")                  \
   V(harmony_simd, "harmony simd")                                            \
   V(harmony_do_expressions, "harmony do-expressions")                        \
   V(harmony_iterator_close, "harmony iterator finalization")                 \
   V(harmony_tailcalls, "harmony tail calls")                                 \
@@ skipping 943 matching lines @@
 #undef DEFINE_ALIAS_FLOAT
 #undef DEFINE_ALIAS_ARGS
 
 #undef FLAG_MODE_DECLARE
 #undef FLAG_MODE_DEFINE
 #undef FLAG_MODE_DEFINE_DEFAULTS
 #undef FLAG_MODE_META
 #undef FLAG_MODE_DEFINE_IMPLICATIONS
 
 #undef COMMA