Reapply "Use a single file for app snapshots."

runtime/bin/extensions_macos.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"
 #if defined(TARGET_OS_MACOS)
 
 #include "bin/extensions.h"
 #include <dlfcn.h>  // NOLINT
 
 namespace dart {
 namespace bin {
 
-const char* kPrecompiledLibraryName = "libprecompiled.dylib";
+const char* kPrecompiledVMIsolateSymbolName = "kVmIsolateSnapshot";
+const char* kPrecompiledIsolateSymbolName = "kIsolateSnapshot";
 const char* kPrecompiledInstructionsSymbolName = "kInstructionsSnapshot";
 const char* kPrecompiledDataSymbolName = "kDataSnapshot";
 
 void* Extensions::LoadExtensionLibrary(const char* library_file) {
   return dlopen(library_file, RTLD_LAZY);
 }
 
 void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
   dlerror();
   return dlsym(lib_handle, symbol);
 }
 
 Dart_Handle Extensions::GetError() {
   const char* err_str = dlerror();
   if (err_str != NULL) {
     return Dart_NewApiError(err_str);
   }
   return Dart_Null();
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)