Merged: Check for encoding when appending in string builder

Fixed: chromium:1450114
(cherry picked from commit a7e2bef)

Change-Id: I2c692c385def56a2ee07e5ae902200249e00d470
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4604097
Reviewed-by: Igor Sheludko <[email protected]>
Cr-Commit-Position: refs/branch-heads/11.5@{v8#8}
Cr-Branched-From: 0c4044b-refs/heads/11.5.150@{#1}
Cr-Branched-From: b71d303-refs/heads/main@{#87781}

src/strings/string-builder.cc

@@ -320,12 +320,21 @@ bool IncrementalStringBuilder::CanAppendByCopy(Handle<String> string) {
 void IncrementalStringBuilder::AppendStringByCopy(Handle<String> string) {
   DCHECK(CanAppendByCopy(string));
 
-  Handle<SeqOneByteString> part =
-      Handle<SeqOneByteString>::cast(current_part());
   {
     DisallowGarbageCollection no_gc;
-    String::WriteToFlat(*string, part->GetChars(no_gc) + current_index_, 0,
-                        string->length());
+    if (encoding_ == String::ONE_BYTE_ENCODING) {
+      String::WriteToFlat(
+          *string,
+          Handle<SeqOneByteString>::cast(current_part())->GetChars(no_gc) +
+              current_index_,
+          0, string->length());
+    } else {
+      String::WriteToFlat(
+          *string,
+          Handle<SeqTwoByteString>::cast(current_part())->GetChars(no_gc) +
+              current_index_,
+          0, string->length());
+    }
   }
   current_index_ += string->length();
   DCHECK(current_index_ <= part_length_);