flag to enable fasta scanner in analyzer

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;
 
 import 'package:front_end/src/scanner/reader.dart';
 
 export 'package:front_end/src/scanner/reader.dart'
     show CharacterReader, CharSequenceReader, SubSequenceReader;
 
 /**
  * A [CharacterReader] that reads a range of characters from another character
  * reader.
  */
 class CharacterRangeReader extends CharacterReader {
   /**
    * The reader from which the characters are actually being read.
    */
   final CharacterReader baseReader;
 
   /**
+   * The first character to be read.
+   */
+  final int startIndex;
+
+  /**
    * The last character to be read.
    */
   final int endIndex;
 
   /**
    * Initialize a newly created reader to read the characters from the given
    * [baseReader] between the [startIndex] inclusive to [endIndex] exclusive.
    */
-  CharacterRangeReader(this.baseReader, int startIndex, this.endIndex) {
+  CharacterRangeReader(this.baseReader, this.startIndex, this.endIndex) {
     baseReader.offset = startIndex - 1;
   }
 
   @override
   int get offset => baseReader.offset;
 
   @override
   void set offset(int offset) {
     baseReader.offset = offset;
   }
 
   @override
   int advance() {
     if (baseReader.offset + 1 >= endIndex) {
       return -1;
     }
     return baseReader.advance();
   }
 
   @override
+  String getContents() =>
+      baseReader.getContents().substring(startIndex, endIndex);
+
+  @override
   String getString(int start, int endDelta) =>
       baseReader.getString(start, endDelta);
 
   @override
   int peek() {
     if (baseReader.offset + 1 >= endIndex) {
       return -1;
     }
     return baseReader.peek();
   }
 }