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

runtime/bin/file_impl.dart

Index: runtime/bin/file_impl.dart
diff --git a/runtime/bin/file_impl.dart b/runtime/bin/file_impl.dart
index 2b08b7322eff1b439f33da130a561dcf02e7ba5e..145e4db197d8746f984d459fa2b03ddd78496ea8 100644
--- a/runtime/bin/file_impl.dart
+++ b/runtime/bin/file_impl.dart
@@ -335,6 +335,32 @@ class _FileUtils {
   static const OSERROR_RESPONSE_ERROR_CODE = 1;
   static const OSERROR_RESPONSE_MESSAGE = 2;
 
+  static List ensureFastAndSerializableBuffer(
+      List buffer, int offset, int 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++;
+      }
+    }
+    return [outBuffer, outOffset];
+  }
+
   static exists(String name) native "File_Exists";
   static open(String name, int mode) native "File_Open";
   static create(String name) native "File_Create";
@@ -349,7 +375,8 @@ class _FileUtils {
       native "File_ReadList";
   static writeByte(int id, int value) native "File_WriteByte";
   static writeList(int id, List<int> buffer, int offset, int bytes) {
-    List result = _ensureFastAndSerializableBuffer(buffer, offset, bytes);
+    List result =
+        _FileUtils.ensureFastAndSerializableBuffer(buffer, offset, bytes);
     List outBuffer = result[0];
     int outOffset = result[1];
     return writeListNative(id, outBuffer, outOffset, bytes);
@@ -919,7 +946,8 @@ class _RandomAccessFile extends _FileBase implements RandomAccessFile {
 
     List result;
     try {
-      result = _ensureFastAndSerializableBuffer(buffer, offset, bytes);
+      result =
+          _FileUtils.ensureFastAndSerializableBuffer(buffer, offset, bytes);
     } catch (e) {
       // Complete asynchronously so the user has a chance to setup
       // handlers without getting exceptions when registering the