Improve scanner performance by remembering string's CharAt function as the string type does not cha…

runtime/vm/scanner.h

Index: runtime/vm/scanner.h
===================================================================
--- runtime/vm/scanner.h	(revision 37049)
+++ runtime/vm/scanner.h	(working copy)
@@ -25,6 +25,8 @@
 // GetStream() scans the entire source text and returns a stream of tokens.
 class Scanner : ValueObject {
  public:
+  typedef int32_t (*CharAtFunc)(const String& str, intptr_t index);
+
   // SourcePosition describes a text location in user friendly
   // terms of line number and column.
   struct SourcePosition {
@@ -75,7 +77,7 @@
   static void InitOnce();
 
   // Return true if str is an identifier.
-  static bool IsIdent(const String& str);
+  bool IsIdent(const String& str);
 
   // Does the token stream contain a valid literal. This is used to implement
   // the Dart methods int.parse and double.parse.
@@ -182,6 +184,8 @@
 
   void ScanScriptTag();
 
+  CharAtFunc CallCharAt() const { return char_at_func_; }
+
   static void PrintTokens(const GrowableTokenStream& ts);
 
   TokenDescriptor current_token_;  // Current token.
@@ -207,6 +211,8 @@
 
   SourcePosition c0_pos_;      // Source position of lookahead character c0_.
 
+  const CharAtFunc char_at_func_;
+
   static KeywordTable keywords_[Token::kNumKeywords];
   static int keywords_char_offset_[kNumLowercaseChars];
 };