enable analyzer scanner adapter generic method support

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 118 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);
+  String getContents() => super.getContents();
 
   @override
   String getString(int start, int endDelta) =>
       super.getString(start - _offsetDelta, endDelta);
 }