When typing out an Attribute, Intellisense doesn't include any available Namespaces other than those included in the System namespace.

This makes traversing Namespaces very difficult when specifying an attribute on a member.

To replicate:
Start typing out an Attribute Usage on a Class, Method, Field, Property, etc.
Look through the Intellisense list for a Namespace that you're looking for (other than System). It doesn't appear.

This issue has been moved from https://developercommunity.visualstudio.com/content/problem/216577/intellisense-doenst-work-in-attributes.html
VSTS ticketId: 584324
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