Broken use before init in constructors

[tjpalmer]

New constructor validation might help this, but here's a problem today:

$ export class Test {
      public x: Float64;
      public y: Float64;
      public constructor() {}
    }
interactive#10: Test
$ export class Test {
      public x: Float64;
      public y: Float64;
    }
2: public x: Float64;
          ⇧
[interactive#11:2+11-12]@S: Property x not initialized in constructor
3: public y: Float64;
          ⇧
[interactive#11:3+11-12]@S: Property y not initialized in constructor
interactive#11: Test

The errors in the latter case are correct and also should be applied to the earlier case.