Assembly references showing as unresolved despite compilation working

**Visual Studio Version**: 16.2.0-pre.1.0+28917.182

**Summary**:

The dependencies node shows several (all?) assembly references as unresolved despite compilation succeeding and no errors/warnings in the error list.

Reported by @poke in https://twitter.com/poke/status/1133495293754920960

**Steps to Reproduce**:

```
$ git clone git@github.com:aspnet/Extensions.git
$ cd Extensions
$ ./restore.cmd
$ ./build.cmd
$ ./startvs.cmd
```

**Expected Behavior**:

Dependencies appear as resolved.

**Actual Behavior**:

Dependencies appear as unresolved. Note that P2P references appear unresolved, though if you dig into them you'll see that ultimately the unresolved leaf nodes are assembly references which are then propagating errors upwards.

**User Impact**:

User is conditioned to not trust the dependencies node, rendering it useless at best and aggravating at worst.