Google started pushing out Chrome 51 earlier today including a change that breaks many apps built on the ArcGIS API for Javascript v3.8 – 3.16 including most notably the “home app” in ArcGIS Online and all Portal for ArcGIS version from 10.3 through 10.4.1
While Esri is working directly with Google to see if the change in question can be reverted before Chrome 51 is too widely distributed, Esri is also preparing patches for all affected products and apps.
More to come as we get updates.