Only allow 'void' as a return type.

lib/compiler/implementation/compiler.dart

Index: lib/compiler/implementation/compiler.dart
===================================================================
--- lib/compiler/implementation/compiler.dart	(revision 12312)
+++ lib/compiler/implementation/compiler.dart	(working copy)
@@ -733,10 +733,16 @@
     backend.codegen(work);
   }
 
-  DartType resolveTypeAnnotation(Element element, TypeAnnotation annotation) {
+  DartType resolveTypeAnnotation(Element element,
+                                 TypeAnnotation annotation) {
     return resolver.resolveTypeAnnotation(element, annotation);
   }
 
+  DartType resolveReturnType(Element element,
+                             TypeAnnotation annotation) {
+    return resolver.resolveReturnType(element, annotation);
+  }
+
   FunctionSignature resolveSignature(FunctionElement element) {
     return withCurrentElement(element,
                               () => resolver.resolveSignature(element));