Separate out validation in Url::fromUri()

Problem/Motivation

The first part of Url::fromUri() does validation on the $uri parameter. It would be useful to be able to run that validation separately from calling Url::fromUri().

Proposed resolution

Create a new method which contains the validation code which gets called by Url::fromUri().

Remaining tasks

Decide what to do. Implement.

User interface changes

None.

API changes

Addition of new method.

Data model changes

None.

Release notes snippet