Use native copy between internal/external bytearrays.

runtime/lib/byte_array.dart

Index: runtime/lib/byte_array.dart
diff --git a/runtime/lib/byte_array.dart b/runtime/lib/byte_array.dart
index 03265f8ca1da2f0e83297d879e3d8a8cf9a0bee6..2ef81e6dfe70951093de3a2dc940b29560cf4da9 100644
--- a/runtime/lib/byte_array.dart
+++ b/runtime/lib/byte_array.dart
@@ -1104,7 +1104,7 @@ class _Uint8Array extends _ByteArrayBase implements Uint8List {
   }
 
   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,
@@ -1710,7 +1710,7 @@ class _ExternalUint8Array extends _ByteArrayBase implements Uint8List {
   }
 
   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,