Speed up identifier, keyword and smi parsing

src/scanner.h

Index: src/scanner.h
diff --git a/src/scanner.h b/src/scanner.h
index 0e3013f4b8f7d6a910dcb9e26042b40238c1aaa2..94f9102c042cddbbffe03865e7c533128017179b 100644
--- a/src/scanner.h
+++ b/src/scanner.h
@@ -565,12 +565,16 @@ class Scanner {
   }
 
   // Low-level scanning support.
-  template <bool capture_raw = false>
+  template <bool capture_raw = false, bool check_surrogate = true>
   void Advance() {
     if (capture_raw) {
       AddRawLiteralChar(c0_);
     }
     c0_ = source_->Advance();
+    if (check_surrogate) HandleLeadSurrugate();
+  }
+
+  void HandleLeadSurrugate() {
     if (unibrow::Utf16::IsLeadSurrogate(c0_)) {
       uc32 c1 = source_->Advance();
       if (!unibrow::Utf16::IsTrailSurrogate(c1)) {