[harmony] Add flat/flatMap to Array.prototype[Symbol.unscopables]

https://github.com/tc39/proposal-flatMap/issues/74
https://github.com/tc39/proposal-flatMap/pull/75

BUG=v8:7220

Change-Id: I93ce66bddbb429beaa7486311f120ec32d6a35fb
Reviewed-on: https://chromium-review.googlesource.com/c/1390125Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58537}

src/bootstrapper.cc

@@ -1776,6 +1776,8 @@ void Genesis::InitializeGlobal(Handle<JSGlobalObject> global_object,
     InstallTrueValuedProperty(isolate_, unscopables, "fill");
     InstallTrueValuedProperty(isolate_, unscopables, "find");
     InstallTrueValuedProperty(isolate_, unscopables, "findIndex");
+    InstallTrueValuedProperty(isolate_, unscopables, "flat");
+    InstallTrueValuedProperty(isolate_, unscopables, "flatMap");
     InstallTrueValuedProperty(isolate_, unscopables, "includes");
     InstallTrueValuedProperty(isolate_, unscopables, "keys");
     InstallTrueValuedProperty(isolate_, unscopables, "values");

test/mjsunit/es6/unscopables.js

@@ -96,6 +96,8 @@ function TestArrayPrototypeUnscopables() {
   var fill = 'local fill';
   var find = 'local find';
   var findIndex = 'local findIndex';
+  var flat = 'local flat';
+  var flatMap = 'local flatMap';
   var keys = 'local keys';
   var values = 'local values';
 
@@ -108,6 +110,8 @@ function TestArrayPrototypeUnscopables() {
     assertEquals('local fill', fill);
     assertEquals('local find', find);
     assertEquals('local findIndex', findIndex);
+    assertEquals('local flat', flat);
+    assertEquals('local flatMap', flatMap);
     assertEquals('local keys', keys);
     assertEquals('local values', values);
     assertEquals(42, toString);