Minor clean-ups.

* Track the length of the dex file in RawDexFile.  This can be used to
  improve its assertions.

* Move a sanity check for MmapCloser into the constructor.

* Add a TODO for moving the base64 openers into test code.

* Make the base64 decoder assume C strings, mention this in a comment.

Change-Id: Ib4d5a43d23ebe8819032763c42c536c8b826723a
diff --git a/src/dex_file.h b/src/dex_file.h
index 61add38..3545c07 100644
--- a/src/dex_file.h
+++ b/src/dex_file.h
@@ -17,6 +17,7 @@
 
   // Opens a .dex file from a base64 encoded array.  Returns NULL on
   // failure.
+  // TODO: move this into the DexFile unit test
   static DexFile* OpenBase64(const char* base64);
 
   // Opens a .dex file from a RawDexFile.  Takes ownership of the