Add option to gen_snapshot for creating a Makefile describing a snapshot's dependencies.

runtime/bin/isolate_data.cc

 // Copyright (c) 2017, 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/isolate_data.h"
 #include "bin/snapshot_utils.h"
 
 namespace dart {
 namespace bin {
 
 IsolateData::IsolateData(const char* url,
                          const char* package_root,
                          const char* packages_file,
                          AppSnapshot* app_snapshot)
     : script_url((url != NULL) ? strdup(url) : NULL),
       package_root(NULL),
       packages_file(NULL),
       udp_receive_buffer(NULL),
       builtin_lib_(NULL),
       loader_(NULL),
-      app_snapshot_(app_snapshot) {
+      app_snapshot_(app_snapshot),
+      dependencies_(NULL) {
   if (package_root != NULL) {
     ASSERT(packages_file == NULL);
     this->package_root = strdup(package_root);
   } else if (packages_file != NULL) {
     this->packages_file = strdup(packages_file);
   }
 }
 
 
 IsolateData::~IsolateData() {
   free(script_url);
   script_url = NULL;
   free(package_root);
   package_root = NULL;
   free(packages_file);
   packages_file = NULL;
   free(udp_receive_buffer);
   udp_receive_buffer = NULL;
   if (builtin_lib_ != NULL) {
     Dart_DeletePersistentHandle(builtin_lib_);
   }
   delete app_snapshot_;
   app_snapshot_ = NULL;
 }
 
 }  // namespace bin
 }  // namespace dart