Use a single file for app snapshots.

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 fdc26238821b7d017e80de5cb75e070df6947362..cdc587625c931d85ec9ffe58e9fa8661c151f8be 100644
--- a/runtime/bin/file_fuchsia.cc
+++ b/runtime/bin/file_fuchsia.cc
@@ -75,7 +75,13 @@ bool File::IsClosed() {
 }
 
 
-void* File::MapExecutable(intptr_t* len) {
+void* File::Map(MapType type, int64_t position, int64_t length) {
+  UNIMPLEMENTED();
+  return NULL;
+}
+
+
+void* File::MapReadExecute(int64_t position, int64_t length) {
   UNIMPLEMENTED();
   return NULL;
 }