Revert hiding of VM-only coreimpl list implementation types. While I

runtime/bin/socket_impl.dart

Index: runtime/bin/socket_impl.dart
diff --git a/runtime/bin/socket_impl.dart b/runtime/bin/socket_impl.dart
index 3cd2c770f1cc6a8b7de41ace2b18d256f6beb8f4..49654130d1132e1aeac09c08c10343097025d226 100644
--- a/runtime/bin/socket_impl.dart
+++ b/runtime/bin/socket_impl.dart
@@ -404,8 +404,28 @@ class _Socket extends _SocketBase implements Socket {
       if ((offset + bytes) > buffer.length) {
         throw new IndexOutOfRangeException(offset + bytes);
       }
-      var fastBuffer = _ensureFastAndSerializableBuffer(buffer, offset, bytes);
-      var result = _writeList(fastBuffer[0], fastBuffer[1], bytes);
+      // When using the Dart C API to access raw data, using a ByteArray is
+      // currently much faster. This function will make a copy of the
+      // supplied List to a ByteArray if it isn't already.
+      List outBuffer;
+      int outOffset = offset;
+      if (buffer is Uint8List || buffer is ObjectArray) {
+        outBuffer = buffer;
+      } else {
+        outBuffer = new Uint8List(bytes);
+        outOffset = 0;
+        int j = offset;
+        for (int i = 0; i < bytes; i++) {
+          int value = buffer[j];
+          if (value is! int) {
+            throw new FileIOException(
+                "List element is not an integer at index $j");
+          }
+          outBuffer[i] = value;
+          j++;
+        }
+      }
+      var result = _writeList(outBuffer, outOffset, bytes);
       if (result is OSError) {
         _reportError(result, "Write failed");
         // If writing fails we return 0 as the number of bytes and