Only allow 'void' as a return type.

lib/compiler/implementation/resolver.dart

Index: lib/compiler/implementation/resolver.dart
===================================================================
--- lib/compiler/implementation/resolver.dart	(revision 12312)
+++ lib/compiler/implementation/resolver.dart	(working copy)
@@ -211,7 +211,9 @@
     return visitor.mapping;
   }
 
-  DartType resolveTypeAnnotation(Element element, TypeAnnotation annotation) {
+  DartType resolveTypeAnnotation(Element element,
+                                 TypeAnnotation annotation,
+                                 {bool isVoidAllowed: false}) {
     if (annotation === null) return compiler.types.dynamicType;
     ResolverVisitor visitor = new ResolverVisitor(compiler, element);
     DartType result = visitor.resolveTypeAnnotation(annotation);
@@ -219,6 +221,9 @@
       // TODO(karklose): warning.
       return compiler.types.dynamicType;
     }
+    if (result == compiler.types.voidType && !isVoidAllowed) {
+      error(annotation, MessageKind.VOID_NOT_ALLOWED);
+    }
     return result;
   }
 
@@ -2372,6 +2377,8 @@
   Element visitIdentifier(Identifier node) {
     Element variables = new VariableListElement.node(currentDefinitions,
         ElementKind.VARIABLE_LIST, enclosingElement);
+    // Ensure a parameter is not typed 'void'.
+    variables.computeType(compiler);
     return new VariableElement(node.source, variables,
         ElementKind.PARAMETER, enclosingElement, node: node);
   }
@@ -2491,7 +2498,8 @@
       requiredParameterCount  = parametersBuilder.length;
       parameters = parametersBuilder.toLink();
     }
-    DartType returnType = compiler.resolveTypeAnnotation(element, returnNode);
+    DartType returnType = compiler.resolveTypeAnnotation(
+        element, returnNode, isVoidAllowed: true);
     return new FunctionSignature(parameters,
                                  visitor.optionalParameters,
                                  requiredParameterCount,