[parser] Use Back2() where appropriate.

src/parsing/scanner.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Features shared by parsing and pre-parsing scanners.
 
 #ifndef V8_PARSING_SCANNER_H_
 #define V8_PARSING_SCANNER_H_
 
 #include "src/allocation.h"
@@ skipping 599 matching lines @@
 
   void PushBack(uc32 ch) {
     if (c0_ > static_cast<uc32>(unibrow::Utf16::kMaxNonSurrogateCharCode)) {
       source_->Back2();
     } else {
       source_->Back();
     }
     c0_ = ch;
   }
 
+  // Same as PushBack(ch1); PushBack(ch2).
+  // - Potentially more efficient as it uses Back2() on the stream.
+  // - Uses char as parameters, since we're only calling it with ASCII chars in
+  //   practice. This way, we can avoid a few edge cases.
+  void PushBack2(char ch1, char ch2) {
+    source_->Back2();
+    c0_ = ch2;
+  }
+
   inline Token::Value Select(Token::Value tok) {
     Advance();
     return tok;
   }
 
   inline Token::Value Select(uc32 next, Token::Value then, Token::Value else_) {
     Advance();
     if (c0_ == next) {
       Advance();
       return then;
@@ skipping 165 matching lines @@
   bool found_html_comment_;
 
   MessageTemplate::Template scanner_error_;
   Location scanner_error_location_;
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_PARSING_SCANNER_H_