Description:
'ion-navbar' is not a known element
Steps to Reproduce:
Tabs project. Add color="primary" to ion-navbar
Output:

My ionic info:
C:\ae\adaept.com\aetabs>ionic info
cli packages: (C:\Users\peter\AppData\Roaming\npm\node_modules)
@ionic/cli-utils : 2.0.0-rc.5
ionic (Ionic CLI) : 4.0.0-rc.5
global packages:
cordova (Cordova CLI) : 8.0.0
local packages:
@angular-devkit/core : 0.5.7
@angular-devkit/schematics : 0.5.7
@angular/cli : 6.0.0-rc.5
@ionic/schematics-angular : 1.0.0-rc.5
Cordova Platforms : none
Ionic Framework : @ionic/angular 4.0.0-alpha.2
System:
NodeJS : v8.11.1
npm : 5.8.0
OS : Windows 10
Environment Variables:
ANDROID_HOME : not set
C:\ae\adaept.com\aetabs>
Other Information:
Replace ion-navbar with ion-toolbar

@peterennis Yep, I made an internal note to add that to our breaking changes document: https://github.com/ionic-team/ionic/blob/master/angular/BREAKING.md
Thanks for bringing it to our attention.
Most helpful comment
Replace ion-navbar with ion-toolbar