WIP [esnext] implement async iteration proposal

src/heap-symbols.h

 // Copyright 2015 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.
 
 #ifndef V8_HEAP_SYMBOLS_H_
 #define V8_HEAP_SYMBOLS_H_
 
 #define INTERNALIZED_STRING_LIST(V)                                \
   V(anonymous_string, "anonymous")                                 \
   V(apply_string, "apply")                                         \
@@ skipping 117 matching lines @@
   V(promise_string, "promise")                                     \
   V(proto_string, "__proto__")                                     \
   V(prototype_string, "prototype")                                 \
   V(Proxy_string, "Proxy")                                         \
   V(query_colon_string, "(?:)")                                    \
   V(RangeError_string, "RangeError")                               \
   V(ReferenceError_string, "ReferenceError")                       \
   V(RegExp_string, "RegExp")                                       \
   V(reject_string, "reject")                                       \
   V(resolve_string, "resolve")                                     \
+  V(return_string, "return")                                       \
   V(script_string, "script")                                       \
   V(second_string, "second")                                       \
   V(setPrototypeOf_string, "setPrototypeOf")                       \
   V(set_string, "set")                                             \
   V(Set_string, "Set")                                             \
   V(source_mapping_url_string, "source_mapping_url")               \
   V(source_string, "source")                                       \
   V(sourceText_string, "sourceText")                               \
   V(source_url_string, "source_url")                               \
   V(stack_string, "stack")                                         \
@@ skipping 78 matching lines @@
 
 #define PUBLIC_SYMBOL_LIST(V)                \
   V(iterator_symbol, Symbol.iterator)        \
   V(intl_fallback_symbol, IntlFallback)      \
   V(match_symbol, Symbol.match)              \
   V(replace_symbol, Symbol.replace)          \
   V(search_symbol, Symbol.search)            \
   V(species_symbol, Symbol.species)          \
   V(split_symbol, Symbol.split)              \
   V(to_primitive_symbol, Symbol.toPrimitive) \
-  V(unscopables_symbol, Symbol.unscopables)
+  V(unscopables_symbol, Symbol.unscopables)  \
+  V(async_iterator_symbol, Symbol.asyncIterator)
 
 // Well-Known Symbols are "Public" symbols, which have a bit set which causes
 // them to produce an undefined value when a load results in a failed access
 // check. Because this behaviour is not specified properly as of yet, it only
 // applies to a subset of spec-defined Well-Known Symbols.
 #define WELL_KNOWN_SYMBOL_LIST(V)                           \
   V(has_instance_symbol, Symbol.hasInstance)                \
   V(is_concat_spreadable_symbol, Symbol.isConcatSpreadable) \
   V(to_string_tag_symbol, Symbol.toStringTag)
 
 #endif  // V8_HEAP_SYMBOLS_H_