Clear StringToken.canonicalizer after each parsing.

pkg/front_end/lib/src/fasta/scanner/string_canonicalizer.dart

 // Copyright (c) 2017, 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 fasta.scanner.string_canonicalizer;
 
 import 'dart:convert';
 
 class Node {
   var /* String | List<int> */ data;
@@ skipping 97 matching lines @@
     if (data is String)
       payload = data.substring(start, end);
     else
       payload = decode(data, start, end, asciiOnly);
     _nodes[index] = new Node(data, start, end, payload, s);
     _count++;
     return payload;
   }
 
   clear() {
+    _size = INITIAL_SIZE;
     _nodes = new List<Node>(_size);
+    _count = 0;
   }
 }