Shared block references aren't updated on import

[jom]

In 5.0's block editor, shared block references (ex. <!-- wp:block {"ref":10} /-->) aren't updated when importing posts. The reference breaks when the shared block has a different post ID on the new site.

Steps to Reproduce

Original Site:

1. Using the block editor, create several pages that reference the same shared block.
2. Export the WXR file with all posts.
3. Note the post ID of the wp_block post for the shared block.

New Site:

1. Make sure the post ID for the shared block is already taken by a different kind of post type. For example, if in the original site the shared block had a post ID of 10, make sure that exists (https://example.com/wp-admin/post.php?post=10&action=edit).
2. Import the WXR file.
3. Attempt to edit the imported pages that reference the shared block.
   --> Expected: Shared block is imported and the IDs of all shared block references are mapped correctly.
   --> Actual: Shared block references are broken and unchanged with the ID from the originating site.