flag to enable fasta scanner in analyzer

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

 // Copyright (c) 2016, 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.
 
 /**
  * An object used by the scanner to read the characters to be scanned.
  */
 abstract class CharacterReader {
   /**
    * The current offset relative to the beginning of the source. Return the
@@ skipping 10 matching lines @@
    */
   void set offset(int offset);
 
   /**
    * Advance the current position and return the character at the new current
    * position.
    */
   int advance();
 
   /**
+   * Return the source to be scanned.
+   */
+  String getContents();
+
+  /**
    * Return the substring of the source code between the [start] offset and the
    * modified current position. The current position is modified by adding the
    * [endDelta], which is the number of characters after the current location to
    * be included in the string, or the number of characters before the current
    * location to be excluded if the offset is negative.
    */
   String getString(int start, int endDelta);
 
   /**
    * Return the character at the current position without changing the current
@@ skipping 40 matching lines @@
 
   @override
   int advance() {
     if (_charOffset >= _stringLength) {
       return -1;
     }
     return _sequence.codeUnitAt(_charOffset++);
   }
 
   @override
+  String getContents() => _sequence;
+
+  @override
   String getString(int start, int endDelta) =>
       _sequence.substring(start, _charOffset + endDelta);
 
   @override
   int peek() {
     if (_charOffset >= _stringLength) {
       return -1;
     }
     return _sequence.codeUnitAt(_charOffset);
   }
@@ skipping 20 matching lines @@
 
   @override
   int get offset => _offsetDelta + super.offset;
 
   @override
   void set offset(int offset) {
     super.offset = offset - _offsetDelta;
   }
 
   @override
+  String getContents() => super.getContents().substring(_offsetDelta);
+
+  @override
   String getString(int start, int endDelta) =>
       super.getString(start - _offsetDelta, endDelta);
 }