[scanner] Add Skip to be used after successful Peek

Change-Id: Ic3df370e2859bf77572b34a314ad8ed17b75b942
Reviewed-on: https://chromium-review.googlesource.com/1183485
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55279}

src/parsing/scanner.cc

@@ -529,7 +529,8 @@ Token::Value Scanner::ScanHtmlComment() {
     PushBack('!');  // undo Advance()
     return Token::LT;
   }
-  Advance();
+  source_->Skip('-');
+  DCHECK_EQ('-', c0_);
 
   found_html_comment_ = true;
   return SkipSingleHTMLComment();
@@ -674,7 +675,7 @@ void Scanner::Scan() {
         if (c0_ == '/') {
           uc32 c = Peek();
           if (c == '#' || c == '@') {
-            Advance();
+            source_->Skip(c);
             Advance();
             token = SkipSourceURLComment();
           } else {
@@ -727,7 +728,7 @@ void Scanner::Scan() {
           token = Token::PERIOD;
           if (c0_ == '.') {
             if (Peek() == '.') {
-              Advance();
+              source_->Skip('.');
               Advance();
               token = Token::ELLIPSIS;
             }
@@ -1004,8 +1005,8 @@ Token::Value Scanner::ScanTemplateSpan() {
       result = Token::TEMPLATE_TAIL;
       break;
     } else if (c == '$' && Peek() == '{') {
-      Advance();  // Consume '$'
-      Advance();  // Consume '{'
+      source_->Skip('{');
+      Advance();
       break;
     } else if (c == '\\') {
       Advance();  // Consume '\\'

src/parsing/scanner.h

@@ -50,6 +50,12 @@ class Utf16CharacterStream {
     }
   }
 
+  inline void Skip(uc32 c) {
+    DCHECK_LT(buffer_cursor_, buffer_end_);
+    DCHECK_EQ(c, static_cast<uc32>(*buffer_cursor_));
+    buffer_cursor_++;
+  }
+
   // Returns and advances past the next UTF-16 code unit in the input
   // stream. If there are no more code units it returns kEndOfInput.
   inline uc32 Advance() {
@@ -634,13 +640,13 @@ class Scanner {
   bool CombineSurrogatePair() {
     DCHECK(!unibrow::Utf16::IsLeadSurrogate(kEndOfInput));
     if (unibrow::Utf16::IsLeadSurrogate(c0_)) {
-      uc32 c1 = source_->Advance();
+      uc32 c1 = source_->Peek();
       DCHECK(!unibrow::Utf16::IsTrailSurrogate(kEndOfInput));
       if (unibrow::Utf16::IsTrailSurrogate(c1)) {
         c0_ = unibrow::Utf16::CombineSurrogatePair(c0_, c1);
+        source_->Skip(c1);
         return true;
       }
-      source_->Back();
     }
     return false;
   }