Describe the bug
Testing 3.8RC1 with one of my plugin, I've found that the block-serialization-default-parser package was missing into the packages folder.

To Reproduce
Steps to reproduce the behavior:

1. Download & Activate https://github.com/imath/gutenblocks/releases/tag/1.2.5
2. Create a new post
3. Type /dubb (or /doubl if you're french) into the default block and select the Dubber (or Doubleur if you're french) block
4. Type anything into one of the nested blocks and wait for the Autosave to throw the error "Updating failed"
5. Here's the output of the rest response:

Warning: require_once(PathTo\gutenberg\lib/../packages/block-serialization-default-parser/parser.php): failed to open stream:
No such file or directory in PathTo\gutenberg\lib\blocks.php on line 79

Looking into the packages folder of the 3.8 Release Candidate 1, there's no block-serialization-default-parser/parser.php file in it. See screenshot below.

Expected behavior
the gutenberg_parse_blocks() function should load the WP_Block_Parser.

Screenshots

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Chrome
• Version: 68

Additional context

• Gutenberg version: https://github.com/WordPress/gutenberg/releases/tag/v3.8.0-rc.1