Reproduction steps:
1. Open project attached (LazyTask.zip).
2. Open scene "Main".
3. Press play.

Expected result: Editor will enter play mode.
Actual result: Error appears: "Assets/LazyTask/Scripts/Main.cs(14,43): error CS0234: The type or namespace name `Tasks' does not exist in the namespace `System.Threading'. Are you missing an assembly reference?"

4. Comment line 14 and line 21 in Main.cs
5. Press play. (It should enter play mode).
6. Press test button in game view. (It should type "Hello" in console).
7. Open PlayerSettings window.

Expected result: PlayerSettings window will open and there will be no other consequences.
Actual result: Error appears: "Assets/LazyTask/Scripts/Main.cs(9,33): error CS0433: The imported type `System.Lazy<T>' is defined multiple times" and it is no longer usable. Unable to enter play mode.

8. Uncomment line 21 and 14 and comment line 9 and 20.
9. Press play.

Expected result: Error "The type or namespace name `Tasks' does not exist in the namespace..." will appear.
Actual Result: Player enter play mode.

Reproduced on: 2017.1.0b1.