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

runtime/bin/file_fuchsia.cc

Index: runtime/bin/file_fuchsia.cc
diff --git a/runtime/bin/file_fuchsia.cc b/runtime/bin/file_fuchsia.cc
index 7310f43b199856125ea966e623815cef4d074e0b..5ed46d503361fc1b5ace624c8aacea1122f052dc 100644
--- a/runtime/bin/file_fuchsia.cc
+++ b/runtime/bin/file_fuchsia.cc
@@ -100,6 +100,27 @@ int64_t File::Write(const void* buffer, int64_t num_bytes) {
 }
 
 
+bool File::VPrint(const char* format, va_list args) {
+  // Measure.
+  va_list measure_args;
+  va_copy(measure_args, args);
+  intptr_t len = vsnprintf(NULL, 0, format, measure_args);
+  va_end(measure_args);
+
+  char* buffer = reinterpret_cast<char*>(malloc(len + 1));
+
+  // Print.
+  va_list print_args;
+  va_copy(print_args, args);
+  vsnprintf(buffer, len + 1, format, print_args);
+  va_end(print_args);
+
+  bool result = WriteFully(buffer, len);
+  free(buffer);
+  return result;
+}
+
+
 int64_t File::Position() {
   ASSERT(handle_->fd() >= 0);
   return NO_RETRY_EXPECTED(lseek(handle_->fd(), 0, SEEK_CUR));