Support checked mode for return type.

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
===================================================================
--- lib/compiler/implementation/lib/js_helper.dart	(revision 14460)
+++ lib/compiler/implementation/lib/js_helper.dart	(working copy)
@@ -1375,6 +1375,8 @@
   propertyTypeCastError(value, property);
 }
 
+voidTypeCheck(value) => identical(value, null);

[kasperl, 2012/11/02 12:53:05]

This may be what the spec says today, but I still think that's it's broken that
we allow code like this:

   void foo() => null;

That was never the intent.

[sra1, 2012/11/02 20:13:51]

We *do* want to allow

   void foo() => bar();

This makes the delegate pattern look nice and uniform.
It is also nice to be able to write

   void foo() => throw UnimplementedError('No foo for you');

It is possible that

   void foo(a) => a + 1;

is correct, if perverse (operator+ declared as void).

What does the editor say?
If the editor does not already call this out, it could.  Any other literal will
give some kind of type warning, and any resolved invocation will warn if the
target is declared with a type other than dynamic.

I think the cure for trying to prevent your foo will be worse than the disease.

It follows from of bar returning null instead of some other marker value:

bar(){}

+
 /**
  * Special interface recognized by the compiler and implemented by DOM
  * objects that support integer indexing. This interface is not