No longer OOM on invalid string length.

src/json-stringifier.h

Index: src/json-stringifier.h
diff --git a/src/json-stringifier.h b/src/json-stringifier.h
index a75b3deed50eacb0863ef2eed4957302674e3ac6..c063b67c08e0c26b53599d24f131f89c182b3303 100644
--- a/src/json-stringifier.h
+++ b/src/json-stringifier.h
@@ -266,6 +266,7 @@ BasicJsonStringifier::BasicJsonStringifier(Isolate* isolate)
                            factory_->ToObject(factory_->empty_string()));
   part_length_ = kInitialPartLength;
   current_part_ = factory_->NewRawOneByteString(part_length_);
+  ASSERT(!current_part_.is_null());
   tojson_string_ = factory_->toJSON_string();
   stack_ = factory_->NewJSArray(8);
 }
@@ -309,6 +310,7 @@ MaybeObject* BasicJsonStringifier::StringifyString(Isolate* isolate,
   if (object->IsOneByteRepresentationUnderneath()) {
     Handle<String> result =
         isolate->factory()->NewRawOneByteString(worst_case_length);
+    ASSERT(!result.is_null());
     DisallowHeapAllocation no_gc;
     return StringifyString_<SeqOneByteString>(
         isolate,
@@ -317,6 +319,7 @@ MaybeObject* BasicJsonStringifier::StringifyString(Isolate* isolate,
   } else {
     Handle<String> result =
         isolate->factory()->NewRawTwoByteString(worst_case_length);
+    ASSERT(!result.is_null());
     DisallowHeapAllocation no_gc;
     return StringifyString_<SeqTwoByteString>(
         isolate,
@@ -722,7 +725,6 @@ void BasicJsonStringifier::ShrinkCurrentPart() {
 void BasicJsonStringifier::Accumulate() {
   if (accumulator()->length() + current_part_->length() > String::kMaxLength) {
     // Screw it.  Simply set the flag and carry on.  Throw exception at the end.
-    // We most likely will trigger a real OOM before even reaching this point.
     set_accumulator(factory_->empty_string());
     overflowed_ = true;
   } else {
@@ -741,6 +743,7 @@ void BasicJsonStringifier::Extend() {
   } else {
     current_part_ = factory_->NewRawTwoByteString(part_length_);
   }
+  ASSERT(!current_part_.is_null());
   current_index_ = 0;
 }
 
@@ -749,6 +752,7 @@ void BasicJsonStringifier::ChangeEncoding() {
   ShrinkCurrentPart();
   Accumulate();
   current_part_ = factory_->NewRawTwoByteString(part_length_);
+  ASSERT(!current_part_.is_null());
   current_index_ = 0;
   is_ascii_ = false;
 }