Convert Unicode code points outside the basic multilingual plane to the replacement character.

src/heap.cc

Index: src/heap.cc
diff --git a/src/heap.cc b/src/heap.cc
index 6ae46f2a6ee743e4bac99446ca61e08288657b91..fa6344556fbcc43e633a1d9c5c67eb7cca7ae3a7 100644
--- a/src/heap.cc
+++ b/src/heap.cc
@@ -2866,6 +2866,8 @@ Object* Heap::AllocateStringFromAscii(Vector<const char> string,
 
 Object* Heap::AllocateStringFromUtf8(Vector<const char> string,
                                      PretenureFlag pretenure) {
+  // V8 only supports characters in the Basic Multilingual Plane.
+  const uc32 kMaxSupportedChar = 0xFFFF;
   // Count the number of characters in the UTF-8 string and check if
   // it is an ASCII string.
   Access<Scanner::Utf8Decoder> decoder(Scanner::utf8_decoder());
@@ -2890,6 +2892,7 @@ Object* Heap::AllocateStringFromUtf8(Vector<const char> string,
   decoder->Reset(string.start(), string.length());
   for (int i = 0; i < chars; i++) {
     uc32 r = decoder->GetNext();
+    if (r > kMaxSupportedChar) { r = unibrow::Utf8::kBadChar; }
     string_result->Set(i, r);
   }
   return result;