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

runtime/bin/isolate_data.cc

Index: runtime/bin/isolate_data.cc
diff --git a/runtime/bin/isolate_data.cc b/runtime/bin/isolate_data.cc
index b37cf178431042b05f922e5e3c17e02a6a74a7d5..c10180df6feae90b4c1563cec5cc9d814beb1a98 100644
--- a/runtime/bin/isolate_data.cc
+++ b/runtime/bin/isolate_data.cc
@@ -18,7 +18,8 @@ IsolateData::IsolateData(const char* url,
       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);