Optimize flattening of cons strings with equal left and right-side.

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index 0be534696765b1aac36f37f2c358d50273b0a6a2..320fbb529c3b072b152f00386d467ea9348a0864 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -11539,6 +11539,10 @@ void String::WriteToFlat(String* src,
           // Right hand side is longer.  Recurse over left.
           if (from < boundary) {
             WriteToFlat(first, sink, from, boundary);
+            if (from == 0 && cons_string->second() == first) {
+              CopyChars(sink + boundary, sink, boundary);
+              return;
+            }
             sink += boundary - from;
             from = 0;
           } else {