Replace the use CharacterStreams in Heap::AllocateSymbolInternal and String::ComputeHash

src/json-parser.h

Index: src/json-parser.h
diff --git a/src/json-parser.h b/src/json-parser.h
index 2f980cc05b22d53e4471952e2d219459bed3fee0..5d2fae4c930a851ee1f0992767e0cafc00c2a33d 100644
--- a/src/json-parser.h
+++ b/src/json-parser.h
@@ -631,7 +631,17 @@ Handle<String> JsonParser<seq_ascii>::ScanJsonString() {
                                                         position_);
       }
       if (c0 < 0x20) return Handle<String>::null();
-      running_hash = StringHasher::AddCharacterCore(running_hash, c0);
+      if (static_cast<uint32_t>(c0) >
+          unibrow::Utf16::kMaxNonSurrogateCharCode) {
+        running_hash =
+            StringHasher::AddCharacterCore(running_hash,
+                                           unibrow::Utf16::LeadSurrogate(c0));
+        running_hash =
+            StringHasher::AddCharacterCore(running_hash,
+                                           unibrow::Utf16::TrailSurrogate(c0));
+      } else {
+        running_hash = StringHasher::AddCharacterCore(running_hash, c0);
+      }
       position++;
       if (position >= source_length_) return Handle<String>::null();
       c0 = seq_source_->SeqOneByteStringGet(position);