Small improvement to scanner performance

pkg/analyzer/lib/src/dart/scanner/reader.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.dart.scanner.reader;
 
 /**
  * A [CharacterReader] that reads a range of characters from another character
  * reader.
  */
@@ skipping 91 matching lines @@
  */
 class CharSequenceReader implements CharacterReader {
   /**
    * The sequence from which characters will be read.
    */
   final String _sequence;
 
   /**
    * The number of characters in the string.
    */
-  int _stringLength = 0;
+  int _stringLength;
 
   /**
-   * The index, relative to the string, of the last character that was read.
+   * The index, relative to the string, of the next character to be read.
    */
-  int _charOffset = 0;
+  int _charOffset;
 
   /**
    * Initialize a newly created reader to read the characters in the given
    * [_sequence].
    */
   CharSequenceReader(this._sequence) {
     this._stringLength = _sequence.length;
-    this._charOffset = -1;
+    this._charOffset = 0;
   }
 
   @override
-  int get offset => _charOffset;
+  int get offset => _charOffset - 1;
 
   @override
   void set offset(int offset) {
-    _charOffset = offset;
+    _charOffset = offset + 1;
   }
 
   @override
   int advance() {
-    if (_charOffset + 1 >= _stringLength) {
+    if (_charOffset >= _stringLength) {
       return -1;
     }
-    return _sequence.codeUnitAt(++_charOffset);
+    return _sequence.codeUnitAt(_charOffset++);
   }
 
   @override
   String getString(int start, int endDelta) =>
-      _sequence.substring(start, _charOffset + 1 + endDelta).toString();
+      _sequence.substring(start, _charOffset + endDelta).toString();
 
   @override
   int peek() {
-    if (_charOffset + 1 >= _stringLength) {
+    if (_charOffset >= _stringLength) {
       return -1;
     }
-    return _sequence.codeUnitAt(_charOffset + 1);
+    return _sequence.codeUnitAt(_charOffset);
   }
 }
 
 /**
  * A [CharacterReader] that reads characters from a character sequence, but adds
  * a delta when reporting the current character offset so that the character
  * sequence can be a subsequence from a larger sequence.
  */
 class SubSequenceReader extends CharSequenceReader {
   /**
@@ skipping 14 matching lines @@
 
   @override
   void set offset(int offset) {
     super.offset = offset - _offsetDelta;
   }
 
   @override
   String getString(int start, int endDelta) =>
       super.getString(start - _offsetDelta, endDelta);
 }