Fix analyzer warnings in StringCanonicalizer

pkg/front_end/lib/src/fasta/scanner/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.abstract_scanner;
 
 import 'dart:convert';
 
 class Node {
   var /* String | List<int> */ data; 
@@ skipping 16 matching lines @@
   /// Mask away top bits to keep hash calculation within 32-bit SMI range.
   static const int MASK = 16 * 1024 * 1024 - 1;
 
   static const int INITIAL_SIZE = 8 * 1024;
 
   /// Linear size of a hash table.
   int _size = INITIAL_SIZE;
   /// Items in a hash table.
   int _count = 0;
   /// The table itself.
-  Node _nodes = new List<Node>(INITIAL_SIZE);
+  List<Node> _nodes = new List<Node>(INITIAL_SIZE);
 
   static String decode(List<int> data, int start, int end, bool asciiOnly) {
     var s;
     if (asciiOnly) {
       s = new String.fromCharCodes(data, start, end);
     } else {
       s = new Utf8Decoder(allowMalformed: true).convert(data, start, end);
     }
     return s;
   }
@@ skipping 61 matching lines @@
       payload = decode(data, start, end, asciiOnly);
     _nodes[index] = new Node(data, start, end, payload, s);
     _count++;
     return payload;
   }
 
   clear() {
     _nodes = new List<Node>(_size);
   }
 }