fix unsound cast failures in tests

tests/lib_strong/convert/unicode_tests.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.
 
 library unicode_tests;
 
 // Google favorite: "Îñţérñåţîöñåļîžåţîờñ".
 const INTER_BYTES = const [
   0xc3,
   0x8e,
@@ skipping 187 matching lines @@
 const BIGGEST_3_UTF8_UNIT_BYTES = const [0xEF, 0xBF, 0xBF];
 const BIGGEST_3_UTF8_UNIT_STRING = "\u{FFFF}";
 
 const SMALLEST_4_UTF8_UNIT_BYTES = const [0xF0, 0x90, 0x80, 0x80];
 const SMALLEST_4_UTF8_UNIT_STRING = "\u{10000}";
 
 const BIGGEST_4_UTF8_UNIT_BYTES = const [0xF4, 0x8F, 0xBF, 0xBF];
 const BIGGEST_4_UTF8_UNIT_STRING = "\u{10FFFF}";
 
 const _TEST_PAIRS = const [
-  const [const [], ""],
+  const [const <int>[], ""],
   const [INTER_BYTES, INTER_STRING],
   const [BLUEBERRY_BYTES, BLUEBERRY_STRING],
   const [SIVA_BYTES1, SIVA_STRING1],
   const [SIVA_BYTES2, SIVA_STRING2],
   const [BEE_BYTES, BEE_STRING],
   const [DIGIT_BYTES, DIGIT_STRING],
   const [ASCII_BYTES, ASCII_STRING],
   const [BIGGEST_ASCII_BYTES, BIGGEST_ASCII_STRING],
   const [SMALLEST_2_UTF8_UNIT_BYTES, SMALLEST_2_UTF8_UNIT_STRING],
   const [BIGGEST_2_UTF8_UNIT_BYTES, BIGGEST_2_UTF8_UNIT_STRING],
   const [SMALLEST_3_UTF8_UNIT_BYTES, SMALLEST_3_UTF8_UNIT_STRING],
   const [BIGGEST_3_UTF8_UNIT_BYTES, BIGGEST_3_UTF8_UNIT_STRING],
   const [SMALLEST_4_UTF8_UNIT_BYTES, SMALLEST_4_UTF8_UNIT_STRING],
   const [BIGGEST_4_UTF8_UNIT_BYTES, BIGGEST_4_UTF8_UNIT_STRING],
 ];
 
 List<List> _expandTestPairs() {
   assert(2 == BEE_STRING.length);
-  var tests = [];
+  var tests = <List>[];
   tests.addAll(_TEST_PAIRS);
   tests.addAll(_TEST_PAIRS.map((test) {
     var bytes = test[0];
     var string = test[1];
-    var longBytes = [];
+    var longBytes = <int>[];
     var longString = "";
     for (int i = 0; i < 100; i++) {
       longBytes.addAll(bytes);
       longString += string;
     }
     return [longBytes, longString];
   }));
   return tests;
 }
 
 final List UNICODE_TESTS = _expandTestPairs();