[esnext] ship --harmony-async-iteration

Enable --harmony-async-iteration (Symbol.asyncIterator, async generator
syntax, and for-await-of syntax) by default, as discussed in
https://groups.google.com/forum/#!topic/v8-users/SlLEsgNv4JY

BUG=v8:5855
R=adamk@chromium.org, gsathya@chromium.org

Change-Id: I77a77124a68813431daceca1b0cbaec5af271fee
Reviewed-on: https://chromium-review.googlesource.com/668877
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48068}

src/flag-definitions.h

@@ -205,7 +205,6 @@ DEFINE_IMPLICATION(es_staging, harmony)
     "harmony disallow non undefined primitive return value from class " \
     "constructor")                                                      \
   V(harmony_dynamic_import, "harmony dynamic import")                   \
-  V(harmony_async_iteration, "harmony async iteration")
 
 #ifdef V8_INTL_SUPPORT
 #define HARMONY_STAGED(V)                        \
@@ -227,6 +226,7 @@ DEFINE_IMPLICATION(es_staging, harmony)
   V(harmony_sharedarraybuffer, "harmony sharedarraybuffer")              \
   V(harmony_regexp_dotall, "harmony regexp dotAll flag")                 \
   V(harmony_regexp_lookbehind, "harmony regexp lookbehind")              \
+  V(harmony_async_iteration, "harmony async iteration")                  \
   V(harmony_template_escapes,                                            \
     "harmony invalid escapes in tagged template literals")               \
   V(harmony_promise_finally, "harmony Promise.prototype.finally")