Fix some WebSocket strong-mode errors and warnings.

sdk/lib/io/bytes_builder.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 /**
  * Builds a list of bytes, allowing bytes and lists of bytes to be added at the
  * end.
  *
@@ skipping 135 matching lines @@
     x |= x >> 4;
     x |= x >> 8;
     x |= x >> 16;
     return x + 1;
   }
 }
 
 
 class _BytesBuilder implements BytesBuilder {
   int _length = 0;
-  final List _chunks = [];
+  final _chunks = <List<int>>[];
 
   void add(List<int> bytes) {
     if (bytes is! Uint8List) {
       bytes = new Uint8List.fromList(bytes);
     }
     _chunks.add(bytes);
     _length += bytes.length;
   }
 
   void addByte(int byte) { add([byte]); }
@@ skipping 30 matching lines @@
 
   bool get isEmpty => _length == 0;
 
   bool get isNotEmpty => _length != 0;
 
   void clear() {
     _length = 0;
     _chunks.clear();
   }
 }