chore(build): add a validation step for angularFiles

[matsko]

The output looks like this (when passing):

Running "validate-angular-files" task
Validating 13 files from the 'ngAnimate' module
Validating 5 files from the 'ngCookies' module
Validating 7 files from the 'ngMessageFormat' module
Validating 3 files from the 'ngMessages' module
Validating 3 files from the 'ngResource' module
Validating 5 files from the 'ngRoute' module
Validating 4 files from the 'ngSanitize' module
Validating 3 files from the 'ngMock' module
Validating 6 files from the 'ngTouch' module
Validating 3 files from the 'ngAria' module
Validating 76 files from the 'ng' module
Validating 5 files from the 'angularLoader' module
All files were detected successfully!

When failing (when a file is referenced in a module but doesn't exist in the local filesystem):

Running "validate-angular-files" task
Validating 14 files from the 'ngAnimate' module
>> src/ngAnimate/referenced-but-missing-file.js does not exist in the local file structure
Validating 5 files from the 'ngCookies' module
Validating 7 files from the 'ngMessageFormat' module
Validating 3 files from the 'ngMessages' module
Validating 3 files from the 'ngResource' module
Validating 5 files from the 'ngRoute' module
Validating 4 files from the 'ngSanitize' module
Validating 3 files from the 'ngMock' module
Validating 6 files from the 'ngTouch' module
Validating 3 files from the 'ngAria' module
Validating 76 files from the 'ng' module
Validating 5 files from the 'angularLoader' module
Warning: Not all files were properly detected the local file structure Use --force to continue.

When failing (when a file exists locally but isn't used in any module):

Running "validate-angular-files" task
Validating 13 files from the 'ngAnimate' module
Validating 5 files from the 'ngCookies' module
Validating 7 files from the 'ngMessageFormat' module
Validating 3 files from the 'ngMessages' module
Validating 3 files from the 'ngResource' module
Validating 5 files from the 'ngRoute' module
Validating 4 files from the 'ngSanitize' module
Validating 3 files from the 'ngMock' module
Validating 6 files from the 'ngTouch' module
Validating 3 files from the 'ngAria' module
Validating 76 files from the 'ng' module
Validating 5 files from the 'angularLoader' module
>> src/ngAnimate/missing-file exists in the local file structure but isn't used by any module
Warning: Not all files were properly detected the local file structure Use --force to continue.

[matsko]

@petebacondarwin and @IgorMinar please review.

[gkalpak]

Not directly related to this PR, but there's a space missing before 'clean' 😁

[petebacondarwin]

This is missing a required change to npm shrinkwrap

[petebacondarwin]

we can use lodash here for assign(), so no actual need for Node 4

[petebacondarwin]

Last thing is that I think the task should be in its own file in lib/grunt/

I have created a new PR with a number of these changes...
#13569