fix unsound cast failures in tests

tests/lib_strong/convert/chunked_conversion_utf87_test.dart

Index: tests/lib_strong/convert/chunked_conversion_utf87_test.dart
diff --git a/tests/lib_strong/convert/chunked_conversion_utf87_test.dart b/tests/lib_strong/convert/chunked_conversion_utf87_test.dart
index d24c6359524d8670dc9abb4e6b9656ae24959547..bfa9869d4e5207c924873e29704ff6111af26955 100644
--- a/tests/lib_strong/convert/chunked_conversion_utf87_test.dart
+++ b/tests/lib_strong/convert/chunked_conversion_utf87_test.dart
@@ -229,37 +229,37 @@ main() {
     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}"
       ]
     ];