Avoid @pragma in documentation

[lrhn]

The VM is adding @pragma annotations to the SDK source (https://dart-review.googlesource.com/c/sdk/+/65545). These must not show up in the API documentation. Pragma annotations are, by definition, only intended for specific tools, not for users, and there is only negative value in exposing them.

[jcollins-g]

This is another "special" object we need to cloak and provide more generic support in Dartdoc for this.

[jcollins-g]

Special object handling was made more generic in #1729 so this should be a little easier now.

[jcollins-g]

@pragmas are popping up in more places and breaking Dartdoc internal tests without either a fix for this or the tests.

https://travis-ci.org/dart-lang/dartdoc/jobs/415682622