Error "Outer Function Context is Needed" when class declared in unittest

[Vijay Nayar]

I've run into an unexpected problem that only seems to happen in 
unittests, but not outside of them. Consider the following 
example:

```
unittest {
   class Ab {
     int a;
     string b;

     static class Builder {
       int _a;
       string _b;
       Builder a(int a) {
         _a = a;
         return this;
       }
       Builder b(string b) {
         _b = b;
         return this;
       }
       Ab build() {
         Ab t = new Ab();
         t.a = _a;
         t.b = _b;
         return t;
       }
     }
   }

   Ab ab = new Ab.Builder()
       .a(1)
       .b("ham")
       .build();
   assert(ab.a == 1);
   assert(ab.b == "ham");
}
```

This fails to compile with the following error:
```
Generating test runner configuration 'builder-test-library' for 
'library' (library).
     Starting Performing "unittest" build using /usr/bin/dmd for 
x86_64.
     Building builder ~master: building configuration 
[builder-test-library]
source/builder.d(58,16): Error: outer function context of 
`builder.__unittest_L41_C1` is needed to `new` nested class 
`builder.__unittest_L41_C1.Ab`
Error /usr/bin/dmd failed with exit code 1.
```

However, if I move the class definition outside of the unittest 
block, then everything works fine:
```
class Ab {
   int a;
   string b;

   static class Builder {
     int _a;
     string _b;
     Builder a(int a) {
       _a = a;
       return this;
     }
     Builder b(string b) {
       _b = b;
       return this;
     }
     Ab build() {
       Ab t = new Ab();
       t.a = _a;
       t.b = _b;
       return t;
     }
   }
}

unittest {
   Ab ab = new Ab.Builder()
       .a(1)
       .b("ham")
       .build();
   assert(ab.a == 1);
   assert(ab.b == "ham");
}
```

```
              Generating test runner configuration 
'builder-test-library' for 'library' (library).
     Starting Performing "unittest" build using /usr/bin/dmd for 
x86_64.
     Building builder ~master: building configuration 
[builder-test-library]
      Linking builder-test-library
      Running builder-test-library
2 modules passed unittests
```

Why is this error only found when declaring a class in the 
unittest?