Fix the snapshotter to generate valid precompilation snapshots

runtime/BUILD.gn

 # Copyright (c) 2014, 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.
 
 # TODO(zra): These build arguments should likely be moved to a gni file that is
 # included in BUILD.gn files that care about the values of the flags. For now,
 # since the GN build only happens as part of a Mojo build there is no need for
 # the indirection.
 declare_args() {
   # Instead of using is_debug, we introduce a different flag for specifying a
@@ skipping 32 matching lines @@
       assert(false)
     }
   }
 
   if (dart_debug) {
     defines += ["DEBUG"]
   } else {
     defines += ["NDEBUG"]
   }
 
+  if (is_ios || is_mac) {
+    defines += ["DART_PRECOMPILER"]
+  }
+
   cflags = [
     "-Werror",
     "-Wall",
     "-Wextra", # Also known as -W.
     "-Wno-unused-parameter",
     "-Wnon-virtual-dtor",
     "-Wvla",
     "-Wno-conversion-null",
     "-Woverloaded-virtual",
     "-g3",
@@ skipping 81 matching lines @@
     "vm:libdart_lib_nosnapshot",
     "vm:libdart_lib",
     "vm:libdart_vm",
     "vm:libdart_platform",
     "third_party/double-conversion/src:libdouble_conversion",
   ]
   sources = [
     "vm/libdart_dependency_helper.cc",
   ]
 }