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

sdk/lib/convert/utf.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.
 
 part of dart.convert;
 
 /**
  * An instance of the default implementation of the [Utf8Codec].
  *
  * This instance provides a convenient access to the most common UTF-8
@@ skipping 437 matching lines @@
           value = _REPLACEMENT_CHARACTER;
         }
         if (!_isFirstCharacter || value != _BOM_CHARACTER) {
           _stringSink.writeCharCode(value);
         }
         _isFirstCharacter = false;
       }
 
       while (i < endIndex) {
         int unit = codeUnits[i++];
-        if (unit <= _ONE_BYTE_LIMIT) {
+        // TODO(floitsch): the way we test we could potentially allow
+        // units that are too large, if they happen to have the
+        // right bit-pattern. (Same is true for the multibyte loop above).
+        // TODO(floitsch): optimize this loop. See:
+        // https://codereview.chromium.org/22929022/diff/1/sdk/lib/convert/utf.dart?column_width=80
+        if (unit < 0) {
+          // TODO(floitsch): should this be unit <= 0 ?
+          if (!_allowMalformed) {
+            throw new FormatException(
+                "Negative UTF-8 code unit: -0x${(-unit).toRadixString(16)}");
+          }
+          _stringSink.writeCharCode(_REPLACEMENT_CHARACTER);
+        } else if (unit <= _ONE_BYTE_LIMIT) {
           _isFirstCharacter = false;
           _stringSink.writeCharCode(unit);
         } else {
           if ((unit & 0xE0) == 0xC0) {
             value = unit & 0x1F;
             expectedUnits = extraUnits = 1;
             continue loop;
           }
           if ((unit & 0xF0) == 0xE0) {
             value = unit & 0x0F;
@@ skipping 18 matching lines @@
       }
       break loop;
     }
     if (expectedUnits > 0) {
       _value = value;
       _expectedUnits = expectedUnits;
       _extraUnits = extraUnits;
     }
   }
 }