5% smaller snapshots by omitting object ids when possible

runtime/vm/snapshot_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 "platform/globals.h"
 
 #include "include/dart_debugger_api.h"
 #include "platform/assert.h"
 #include "vm/bigint_operations.h"
 #include "vm/class_finalizer.h"
@@ skipping 2699 matching lines @@
 
   result = Dart_RunLoop();
   EXPECT(Dart_IsError(result));
   EXPECT(Dart_ErrorHasException(result));
   EXPECT_SUBSTRING("Exception: nulltruefalse123456æøå3.14[]100123456789\n",
                    Dart_GetError(result));
 
   Dart_ExitScope();
 }
 
+
+TEST_CASE(OmittedObjectEncodingLength) {
+  StackZone zone(Isolate::Current());
+  uint8_t* buffer;
+  MessageWriter writer(&buffer, &zone_allocator);
+  writer.WriteInlinedObjectHeader(kOmittedObjectId);
+  // For performance, we'd like single-byte headers when ids are omitted.
+  // If this starts failing, consider renumbering the snapshot ids.
+  EXPECT_EQ(1, writer.BytesWritten());
+}
+
 }  // namespace dart