Use native copy between internal/external bytearrays.

runtime/lib/byte_array.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.
 
 /**
  * A random-access sequence of bytes that also provides random access to
  * the fixed-width integers and floating point numbers represented by
  * those bytes. Byte arrays may be used to pack and unpack data from
  * external sources (such as networks or files systems), and to process
  * large quantities of numerical data more efficiently than would be possible
@@ skipping 1086 matching lines @@
   }
 
   List<int> getRange(int start, int length) {
     _rangeCheck(this.length, start, length);
     List<int> result = _new(length);
     result.setRange(0, length, this, start);
     return result;
   }
 
   void setRange(int start, int length, List<int> from, [int startFrom = 0]) {
-    if (from is _Uint8Array) {
+    if (from is _Uint8Array || from is _ExternalUint8Array) {
       _setRange(start * _BYTES_PER_ELEMENT,
                 length * _BYTES_PER_ELEMENT,
                 from,
                 startFrom * _BYTES_PER_ELEMENT);
     } else {
       Arrays.copy(from, startFrom, this, start, length);
     }
   }
 
   String toString() {
@@ skipping 585 matching lines @@
   }
 
   List<int> getRange(int start, int length) {
     _rangeCheck(this.length, start, length);
     List<int> result = new Uint8List(length);
     result.setRange(0, length, this, start);
     return result;
   }
 
   void setRange(int start, int length, List<int> from, [int startFrom = 0]) {
-    if (from is _ExternalUint8Array) {
+    if (from is _ExternalUint8Array || from is _Uint8Array) {
       _setRange(start * _BYTES_PER_ELEMENT,
                 length * _BYTES_PER_ELEMENT,
                 from,
                 startFrom * _BYTES_PER_ELEMENT);
     } else {
       Arrays.copy(from, startFrom, this, start, length);
     }
   }
 
   String toString() {
@@ skipping 1332 matching lines @@
     }
     _rangeCheck(this.length, start, length);
     return _array.subByteArray(_offset + start, length);
   }
 
   static final int _BYTES_PER_ELEMENT = 8;
   final ByteArray _array;
   final int _offset;
   final int _length;
 }