fix unsound cast failures in tests

tests/lib_strong/convert/chunked_conversion_utf83_test.dart

 // Copyright (c) 2012, 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.
 
 // 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.
 
 import "package:expect/expect.dart";
 import 'dart:convert';
 
 String decode(List<int> bytes, int chunkSize) {
   StringBuffer buffer = new StringBuffer();
   var stringSink = new StringConversionSink.fromStringSink(buffer);
   var byteSink = new Utf8Decoder().startChunkedConversion(stringSink);
   int i = 0;
   while (i < bytes.length) {
-    List nextChunk = [];
+    var nextChunk = <int>[];
     for (int j = 0; j < chunkSize; j++) {
       if (i < bytes.length) {
         nextChunk.add(bytes[i]);
         i++;
       }
     }
     byteSink.add(nextChunk);
   }
   byteSink.close();
   return buffer.toString();
 }
 
 String decodeAllowMalformed(List<int> bytes, int chunkSize) {
   StringBuffer buffer = new StringBuffer();
   var stringSink = new StringConversionSink.fromStringSink(buffer);
   var decoder = new Utf8Decoder(allowMalformed: true);
   var byteSink = decoder.startChunkedConversion(stringSink);
   int i = 0;
   while (i < bytes.length) {
-    List nextChunk = [];
+    var nextChunk = <int>[];
     for (int j = 0; j < chunkSize; j++) {
       if (i < bytes.length) {
         nextChunk.add(bytes[i]);
         i++;
       }
     }
     byteSink.add(nextChunk);
   }
   byteSink.close();
   return buffer.toString();
@@ skipping 19 matching lines @@
   Expect.equals("", decodeAllowMalformed([0xEF, 0xBB, 0xBF], 4));
   Expect.equals("a\u{FEFF}", decode([0x61, 0xEF, 0xBB, 0xBF], 1));
   Expect.equals("a\u{FEFF}", decode([0x61, 0xEF, 0xBB, 0xBF], 2));
   Expect.equals("a\u{FEFF}", decode([0x61, 0xEF, 0xBB, 0xBF], 3));
   Expect.equals("a\u{FEFF}", decode([0x61, 0xEF, 0xBB, 0xBF], 4));
   Expect.equals("a\u{FEFF}", decodeAllowMalformed([0x61, 0xEF, 0xBB, 0xBF], 1));
   Expect.equals("a\u{FEFF}", decodeAllowMalformed([0x61, 0xEF, 0xBB, 0xBF], 2));
   Expect.equals("a\u{FEFF}", decodeAllowMalformed([0x61, 0xEF, 0xBB, 0xBF], 3));
   Expect.equals("a\u{FEFF}", decodeAllowMalformed([0x61, 0xEF, 0xBB, 0xBF], 4));
 }