This has happened to me several times:
What’s ultimately solved things for me (OS X) is deleting pub’s cache:
sudo rm -Rf /Users/<username>/.pub-cache
Also, delete the packages directory in your project’s root:
sudo rm -Rf .packages
Then pub get again in your project(s), but make sure that you are not operating as root (a whoami will tell you, and an exit will get you out of there).