Up-to-date check fails with Xaml Islands projects

**Visual Studio Version**: 16.4.0 Preview 2

**Summary**:
When a .NET Core 3.0 WPF/WinForms project references Xaml Islands the up-to-date check does not detect changes in the UWP projects.

**Steps to Reproduce**:

1. Open this [sample code](https://github.com/marb2000/XamlIslands/tree/master/1903_Hands_On_Labs/Exercise01/WPF_XAMLIslands_v1)

2. Build All

3. Change a UWP xaml file, build

**Expected Behavior**:
Change is visible in the running app.
**Actual Behavior**:
Change is not built.

**User Impact**:
User has to rebuild all to see any changes.