[bigint] Adapt Object::TypeOf.

R=jkummerow@chromium.org

Bug: v8:6791
Change-Id: I6ebd14d39666e8ebe8af42f6dfe579e3fd375754
Reviewed-on: https://chromium-review.googlesource.com/711843
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48497}

src/objects.cc

@@ -641,7 +641,7 @@ Handle<String> Object::TypeOf(Isolate* isolate, Handle<Object> object) {
   }
   if (object->IsString()) return isolate->factory()->string_string();
   if (object->IsSymbol()) return isolate->factory()->symbol_string();
-  if (object->IsString()) return isolate->factory()->string_string();
+  if (object->IsBigInt()) return isolate->factory()->bigint_string();
   if (object->IsCallable()) return isolate->factory()->function_string();
   return isolate->factory()->object_string();
 }

test/mjsunit/harmony/bigint.js

@@ -24,8 +24,10 @@ const six = BigInt(6);
 {
   assertEquals(typeof zero, "bigint");
   assertEquals(typeof one, "bigint");
-}
-{
+}{
+  assertEquals(%Typeof(zero), "bigint");
+  assertEquals(%Typeof(one), "bigint");
+}{
   // TODO(neis): Enable once --no-opt can be removed.
   //
   // function Typeof(x) { return typeof x }