I get the same on my Samus.  It appears that dev_install is trying to get this url inside configure_emgerge:

https://commondatastorage.googleapis.com/chromeos-dev-installer/board/samus/canary-R68-10718.13.0/packages/package.provided

For me, that also fails when I paste it into a browser on the same device, with:

Anonymous caller does not have storage.objects.get access to chromeos-dev-installer/board/samus/canary-R68-10718.13.0/packages/package.provided.

This also occurs on peachpi, snow, daisy, minnie and nyan_big.  So I'd wager its broken across the board(s).

I got the same error on my ChromeBook which version is R67-10575.55.0

I found the error was caused by :
curl https://commondatastorage.googleapis.com/chromeos-dev-installer/board/eve/canary-R67-10575.55.0/packages/package.provided

<?xml version='1.0' encoding='UTF-8'?><Error><Code>AccessDenied</Code><Message>Access denied.</Message><Details>Anonymous caller does not have storage.objects.get access to chromeos-dev-installer/board/eve/canary-R67-10575.55.0/packages/package.provided.</Details></Error>

Same error on lulu 10840.0.0

Same on link (R67-10575.54.0)

Here's a workaround that I came up with, which works for me on my Link device.  I am not a ChromiumOS developer and I can't warranty this in any way, so please use this at your own risk:

# Create symlinks for things emerge wants to find in /usr, not /usr/local
ln -s /usr/local/share/portage /usr/share/
ln -s /usr/local/lib/portage /usr/lib/

# Create a simple python-exec replacement which is less discerning
mkdir /usr/lib/python-exec
echo '#!/bin/bash' > /usr/lib/python-exec/python-exec2-c
echo 'base=$(basename "$1")' >> /usr/lib/python-exec/python-exec2-c
echo 'shift' >> /usr/lib/python-exec/python-exec2-c
echo 'exec /usr/local/bin/python2.7 /usr/local/lib/python-exec/python2.7/"$base" "$@"' >> /usr/lib/python-exec/python-exec2-c
chmod 755 /usr/lib/python-exec/python-exec2-c

# Create symlinks for python so that devserver.py can start properly
ln -s /usr/local/bin/python2.7 /usr/bin/python2.7
ln -s /usr/local/bin/python2.7 /usr/bin/python2
ln -s /usr/local/bin/python2.7 /usr/bin/python

# Redo dev_install:
dev_install --reinstall --yes

SJB's rec seems to have worked for me!

>>> No outdated packages were found on your system.
dev_install done. Enjoy!

I gave SJB's recommendation a try, but unfortunately it failed for me.  I'll just wait for the tool to get fixed and try it again at a later date.

Acer C720p, Chromeos 67.0.3396.99
Failure messages in case anyone is interested:
>>> Installing (49 of 77) dev-python/six-1.10.0::portage-stable to /usr/local/
 * Running stacked hooks for pre_pkg_preinst
 *    wrap_old_config_scripts ...                                                                                                               [ ok ]
/usr/local/var/tmp/portage/dev-python/six-1.10.0/temp/environment: line 3279: /usr/bin/python2.7: No such file or directory
 * ERROR: dev-python/six-1.10.0::portage-stable failed (preinst phase):
 *   (no error message)
 * 
 * Call stack:
 *     ebuild.sh, line  134:  Called pkg_preinst
 *   environment, line 2861:  Called python_foreach_impl '_cleanup'
 *   environment, line 3520:  Called multibuild_foreach_variant '_python_multibuild_wrapper' '_cleanup'
 *   environment, line 2522:  Called _multibuild_run '_python_multibuild_wrapper' '_cleanup'
 *   environment, line 2520:  Called _python_multibuild_wrapper '_cleanup'
 *   environment, line  629:  Called python_wrapper_setup
 *   environment, line 3919:  Called python_export 'python2.7' 'PYTHON_CONFIG'
 *   environment, line 3279:  Called die
 * The specific snippet of code:
 *                           flags=$("${PYTHON}" -c 'import sysconfig; print(sysconfig.get_config_var("ABIFLAGS") or "")') || die;
 * 
 * If you need support, post the output of `emerge --info '=dev-python/six-1.10.0::portage-stable'`,
 * the complete build log and the output of `emerge -pqv '=dev-python/six-1.10.0::portage-stable'`.
 * The complete build log is located at '/usr/local/var/tmp/portage/dev-python/six-1.10.0/temp/build.log'.
 * The ebuild environment file is located at '/usr/local/var/tmp/portage/dev-python/six-1.10.0/temp/environment'.
 * Working directory: '/usr/local/lib64/python2.7/site-packages'
 * S: '/usr/local/var/tmp/portage/dev-python/six-1.10.0/work/six-1.10.0'
!!! FAILED preinst: 1

>>> Failed to install dev-python/six-1.10.0 to /usr/local/, Log file:

>>>  '/usr/local/var/tmp/portage/dev-python/six-1.10.0/temp/build.log'

Hi I am not able to create the symbolic link as it complains

ln: failed to create symbolic link '/usr/share/portage': Read-only file system

Thanks

Well I know how to remove read-only. Reboot needed at the end

sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification

and 

sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification --partitions 4

seems to have worked for me. 
Make note, I had to do this before:

"
sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification

and 

sudo /usr/share/vboot/bin/make_dev_ssd.sh --remove_rootfs_verification --partitions 4
"
then reboot, THEN 


I payed special attention to these lines:

"
ln -s /usr/local/bin/python2.7 /usr/bin/python2
ln -s /usr/local/bin/python2.7 /usr/bin/python
"

don't autocomplete /usr/bin/python2 to /usr/bin/python2.7

I did get a warning after running:
"
ln -s /usr/local/bin/python2.7 /usr/bin/python
"
but, dev_install --reinstall --yes seems to have worked...

>> Comment 9 

Thank you
Thanks to your workaround, I could successfully install.
70.0.3538.41 (Official Build) beta @ ASUS C300MA

After dev_install.sh runs, /usr/local/lib/python-exec/python-exec2 has the wrong shebang - 
#!/usr/lib/python-exec/python-exec2
should be
#!/usr/local/lib/python-exec/python-exec2
(note missing .../local/.. path component).

The python-exec2 script uses the wrong path too
   __file__ = os.path.join('/usr/lib/python-exec', EPYTHON,
should be 
   __file__ = os.path.join('/usr/local/lib/python-exec', EPYTHON,

Next, python-exec2-c fails to find the installed python2.7 and so a replacement like the one from comment #9 above works much better.

After that, python will start, but the Portage/emerge installation isn't configured correctly either. It's missing .../local/... all over the place as well. (still working on sorting out this breakage).