Use a single file for app snapshots.

runtime/bin/file.h

Index: runtime/bin/file.h
diff --git a/runtime/bin/file.h b/runtime/bin/file.h
index 6b22793ec5a45626ca3d73e83ebb36fa766b2865..c30c02d8bf2d68874601cc4d073d9f09644543e2 100644
--- a/runtime/bin/file.h
+++ b/runtime/bin/file.h
@@ -88,7 +88,11 @@ class File : public ReferenceCounted<File> {
 
   intptr_t GetFD();
 
-  void* MapExecutable(intptr_t* num_bytes);
+  enum MapType {
+    kReadOnly = 0,
+    kReadExecute = 1,
+  };
+  void* Map(MapType type, int64_t position, int64_t length);
 
   // Read/Write attempt to transfer num_bytes to/from buffer. It returns
   // the number of bytes read/written.