Treat negative utf8-units the same way as other broken encodings.

tests/lib/convert/utf82_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.
 
 library utf8_test;
 import "package:expect/expect.dart";
 import 'dart:convert';
 
 String decode(List<int> bytes) => new Utf8Decoder().convert(bytes);
 String decodeAllowMalformed(List<int> bytes) {
@@ skipping 32 matching lines @@
   [ 0xF9 ],
   [ 0xFA ],
   [ 0xFB ],
   [ 0xFC ],
   [ 0xFD ],
   [ 0xFE ],
   [ 0xFF ],
   [ 0xC0, 0x80 ],
   [ 0xC1, 0x80 ],
   // Outside valid range.
-  [ 0xF4, 0xBF, 0xBF, 0xBF ]];
+  [ 0xF4, 0xBF, 0xBF, 0xBF ],
+  // Negative
+  [ -0x01 ],
+  [ -0xFF ],
+  [ -0x80000000 ],
+  [ -0x40000000 ],
+  [ -0x80000000000000000]];
 
 final TESTS2 = [
   // Test that 0xC0|1, 0x80 does not eat the next character.
   [[ 0xC0, 0x80, 0x61 ], "Xa" ],
   [[ 0xC1, 0x80, 0x61 ], "Xa" ],
   // 0xF5 .. 0xFF never appear in valid UTF-8 sequences.
   [[ 0xF5, 0x80 ], "XX" ],
   [[ 0xF6, 0x80 ], "XX" ],
   [[ 0xF7, 0x80 ], "XX" ],
   [[ 0xF8, 0x80 ], "XX" ],
@@ skipping 61 matching lines @@
     Expect.throws(() => decode3(bytes), (e) => e is FormatException);
     Expect.throws(() => decode4(bytes), (e) => e is FormatException);
 
     String expected = test[1];
     Expect.equals(expected, decodeAllowMalformed(bytes));
     Expect.equals(expected, decodeAllowMalformed2(bytes));
     Expect.equals(expected, decodeAllowMalformed3(bytes));
     Expect.equals(expected, decodeAllowMalformed4(bytes));
   }
 }