Remove an unnecessary invocation of toString

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 125 matching lines @@
   @override
   int advance() {
     if (_charOffset >= _stringLength) {
       return -1;
     }
     return _sequence.codeUnitAt(_charOffset++);
   }
 
   @override
   String getString(int start, int endDelta) =>
-      _sequence.substring(start, _charOffset + endDelta).toString();
+      _sequence.substring(start, _charOffset + endDelta);
 
   @override
   int peek() {
     if (_charOffset >= _stringLength) {
       return -1;
     }
     return _sequence.codeUnitAt(_charOffset);
   }
 }
 
@@ skipping 21 matching lines @@
 
   @override
   void set offset(int offset) {
     super.offset = offset - _offsetDelta;
   }
 
   @override
   String getString(int start, int endDelta) =>
       super.getString(start - _offsetDelta, endDelta);
 }