Commits on Oct 21, 2015

1. Improve checking for null type …

   This commit should fix most cases where the null type leaks to the
   backend (which causes a crash) by catching it in the typechecker
   instead. It generalizes checking for `null` by introducing an auxilary
   function `typecheckNotNull` which is called when an expression is used
   where no additional type information could coerce the null type to a
   reference type (for example the right hand sides of the declarations of
   a let or the body of a closure). It unifies all earlier error messages
   of type "Cannot infer type of null valued expression".
   
   It's usage is not always obvious. For example, when typechecking a
   sequence, all expressions but the last should be checked for the null
   type, but the last one should not, as it's actual type could be inferred
   from the context of the sequence.
   
   There is also added support for propagating an inferred result type. For
   example, in the function
   ```
   def foo() : Foo{
     let x = 42 in
       while x < 0 {
         x = x - 1;
         null;
       }
     }
   ```
   we know that the `null` should be a `Foo` from the result type.

   

   EliasC committed Oct 21, 2015
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