fix unsound cast failures in tests

tests/lib_strong/convert/chunked_conversion_utf86_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.
 
 import "package:expect/expect.dart";
 import 'dart:convert';
 
 String decode(List<int> bytes) {
   StringBuffer buffer = new StringBuffer();
   var stringSink = new StringConversionSink.fromStringSink(buffer);
@@ skipping 190 matching lines @@
   ]
 ];
 
 main() {
   var allTests = TESTS.expand((test) {
     // Pairs of test and expected string output when malformed strings are
     // allowed. Replacement character: U+FFFD
     return [
       [test, "\u{FFFD}"],
       [
-        new List.from([0x61])..addAll(test),
+        new List<int>.from([0x61])..addAll(test),
         "a\u{FFFD}"
       ],
       [
-        new List.from([0x61])
+        new List<int>.from([0x61])
           ..addAll(test)
           ..add(0x61),
         "a\u{FFFD}a"
       ],
-      [new List.from(test)..add(0x61), "\u{FFFD}a"],
-      [new List.from(test)..addAll(test), "\u{FFFD}\u{FFFD}"],
+      [new List<int>.from(test)..add(0x61), "\u{FFFD}a"],
+      [new List<int>.from(test)..addAll(test), "\u{FFFD}\u{FFFD}"],
       [
-        new List.from(test)
+        new List<int>.from(test)
           ..add(0x61)
           ..addAll(test),
         "\u{FFFD}a\u{FFFD}"
       ],
       [
-        new List.from([0xc3, 0xa5])..addAll(test),
+        new List<int>.from([0xc3, 0xa5])..addAll(test),
         "å\u{FFFD}"
       ],
       [
-        new List.from([0xc3, 0xa5])..addAll(test)..addAll([0xc3, 0xa5]),
+        new List<int>.from([0xc3, 0xa5])..addAll(test)..addAll([0xc3, 0xa5]),
         "å\u{FFFD}å"
       ],
       [
-        new List.from(test)..addAll([0xc3, 0xa5]),
+        new List<int>.from(test)..addAll([0xc3, 0xa5]),
         "\u{FFFD}å"
       ],
       [
-        new List.from(test)..addAll([0xc3, 0xa5])..addAll(test),
+        new List<int>.from(test)..addAll([0xc3, 0xa5])..addAll(test),
         "\u{FFFD}å\u{FFFD}"
       ]
     ];
   });
 
   var allTests2 = TESTS2.map((test) {
     // Pairs of test and expected string output when malformed strings are
     // allowed. Replacement character: U+FFFD
     String expected = (test[1] as String).replaceAll("X", "\u{FFFD}");
     return [test[0], expected];
   });
 
   for (var test in []..addAll(allTests)..addAll(allTests2)) {
     List<int> bytes = test[0];
     Expect.throws(() => decode(bytes), (e) => e is FormatException);
 
     String expected = test[1];
     Expect.equals(expected, decodeAllowMalformed(bytes));
   }
 }