Unfortunately this cannot be done cleanly, because <paper-radio-group> has bower-dependency on <paper-radio-button> which I believe is a mistake, since <paper-radio-button> is imported but never actually used in <paper-radio-group> code.

I spoke to Polymer team about this, and they are hesitant to remove this import because they think some users are importing <paper-radio-group> only, while using both -group and -button (so paper-radio-group is serving as a proxy-import). Since this is a breaking change, they also don't want to roll the minor version since some users could be using ^ in bower.

Rolling to major 3.x.x is not an option either, since they've already started making a 3.x.x that's tied to Polymer 3.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9b49fe1afdd230cb0531d053085ba571d061db80

commit 9b49fe1afdd230cb0531d053085ba571d061db80
Author: Scott Chen <scottchen@chromium.org>
Date: Thu May 17 22:47:32 2018

WebUI Polymer: Remove paper-radio-button from third_party/polymer.

Bug:  843701 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id0fcca7511cc0aea7888f83e0f1dcfca77d06a65
Reviewed-on: https://chromium-review.googlesource.com/1062589
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Scott Chen <scottchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559714}
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/bower.json
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/chromium.patch
[delete] https://crrev.com/2bdf223551af93d2edd140f4efb72bd5dae5c4d1/third_party/polymer/v1_0/components-chromium/paper-radio-button/BUILD.gn
[delete] https://crrev.com/2bdf223551af93d2edd140f4efb72bd5dae5c4d1/third_party/polymer/v1_0/components-chromium/paper-radio-button/bower.json
[delete] https://crrev.com/2bdf223551af93d2edd140f4efb72bd5dae5c4d1/third_party/polymer/v1_0/components-chromium/paper-radio-button/compiled_resources2.gyp
[delete] https://crrev.com/2bdf223551af93d2edd140f4efb72bd5dae5c4d1/third_party/polymer/v1_0/components-chromium/paper-radio-button/paper-radio-button-extracted.js
[delete] https://crrev.com/2bdf223551af93d2edd140f4efb72bd5dae5c4d1/third_party/polymer/v1_0/components-chromium/paper-radio-button/paper-radio-button.html
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/components-chromium/paper-radio-group/BUILD.gn
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/components-chromium/paper-radio-group/compiled_resources2.gyp
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/components-chromium/paper-radio-group/paper-radio-group.html
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/components_summary.txt
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/create_components_summary.py
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/third_party/polymer/v1_0/rsync_exclude.txt
[modify] https://crrev.com/9b49fe1afdd230cb0531d053085ba571d061db80/ui/webui/resources/polymer_resources.grdp

paper-radio-group has been removed from third_party at https://chromium-review.googlesource.com/c/chromium/src/+/1277526. This is no longer an issue.