Registered assembly is not shown in Extension tab of Reference dialog for .Net Core project

**Visual Studio Version**: 16.3.0 Preview 2.0[29123.165.master]

**Summary**:
Registered assembly is not shown in Extension tab of Reference dialog for .Net Core project

**Steps to Reproduce**:

1. Create a C# Class Library project named A

2. Register the assembly built by A

3. Create a C# .Net Core project named B, targeting to framework 4.6

4. Open the Reference dialog for B

**Expected Behavior**: There is an assembly named A in the Extension tab of Reference dialog

**Actual Behavior**: There is noting in the Extension tab of Reference dialog

**User Impact**: