Fixes leak of native File objects.

runtime/bin/file_test.cc

 // 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.
 
 #include "bin/file.h"
 #include "platform/assert.h"
 #include "platform/globals.h"
 #include "vm/unit_test.h"
 
 namespace dart {
@@ skipping 14 matching lines @@
 TEST_CASE(Read) {
   const char* kFilename = GetFileName("runtime/bin/file_test.cc");
   File* file = File::Open(kFilename, File::kRead);
   EXPECT(file != NULL);
   char buffer[16];
   buffer[0] = '\0';
   EXPECT(file->ReadFully(buffer, 13));  // ReadFully returns true.
   buffer[13] = '\0';
   EXPECT_STREQ("// Copyright ", buffer);
   EXPECT(!file->WriteByte(1));  // Cannot write to a read-only file.
-  delete file;
+  file->Release();
 }
 
 
 TEST_CASE(FileLength) {
   const char* kFilename =
       GetFileName("runtime/tests/vm/data/fixed_length_file");
   File* file = File::Open(kFilename, File::kRead);
   EXPECT(file != NULL);
   EXPECT_EQ(42, file->Length());
-  delete file;
+  file->Release();
 }
 
 
 TEST_CASE(FilePosition) {
   char buf[42];
   const char* kFilename =
       GetFileName("runtime/tests/vm/data/fixed_length_file");
   File* file = File::Open(kFilename, File::kRead);
   EXPECT(file != NULL);
   EXPECT(file->ReadFully(buf, 12));
   EXPECT_EQ(12, file->Position());
   EXPECT(file->ReadFully(buf, 6));
   EXPECT_EQ(18, file->Position());
-  delete file;
+  file->Release();
 }
 
 }  // namespace bin
 }  // namespace dart