Looks like a user was having trouble installing Dart on Linux.
From angulardart/angular#1788:
Hi, I am trying to install angulardart from https://webdev.dartlang.org/guides/get-started (step 2), but ‘sudo apt-get update’ results in the following and I am unable to install :
W: GPG error: https://storage.googleapis.com/download.dartlang.org/linux/debian stable Release: The following signatures were invalid: EXPKEYSIG 1397BC53640DB551 Google Inc. (Linux Packages Signing Authority) linux-packages-keymaster@google.com
E: The repository ‘https://storage.googleapis.com/download.dartlang.org/linux/debian stable Release’ is not signed.
N: Updating from such a repository can’t be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
I am still getting the same error
An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://storage.googleapis.com/download.dartlang.org/linux/debian stable Release: The following signatures were invalid: EXPKEYSIG 1397BC53640DB551 Google Inc. (Linux Packages Signing Authority) linux-packages-keymaster@google.comFailed to fetch https://storage.googleapis.com/download.dartlang.org/linux/debian/dists/stable/Release.gpg The following signatures were invalid: EXPKEYSIG 1397BC53640DB551 Google Inc. (Linux Packages Signing Authority) linux-packages-keymaster@google.comSome index files failed to download. They have been ignored, or old ones used instead.
I was also having the issue after following the linked steps in Ubuntu 18.10. What fixed it for me was: