Only allow 'void' as a return type.

lib/compiler/implementation/resolver.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 abstract class TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   DartType getType(TypeAnnotation annotation);
   bool isParameterChecked(Element element);
 }
@@ skipping 193 matching lines @@
   }
 
   TreeElements resolveParameter(Element element) {
     Node tree = element.parseNode(compiler);
     ResolverVisitor visitor =
         new ResolverVisitor(compiler, element.enclosingElement);
     initializerDo(tree, visitor.visit);
     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);
     if (result === null) {
       // TODO(karklose): warning.
       return compiler.types.dynamicType;
     }
+    if (result == compiler.types.voidType && !isVoidAllowed) {
+      error(annotation, MessageKind.VOID_NOT_ALLOWED);
+    }
     return result;
   }
 
   /**
    * Load and resolve the supertypes of [cls].
    *
    * Warning: do not call this method directly. It should only be
    * called by [resolveClass] and [ClassSupertypeResolver].
    */
   void loadSupertypes(ClassElement cls, Node from) {
@@ skipping 2133 matching lines @@
     }
     currentDefinitions = node;
     Element element = definition.accept(this);
     currentDefinitions = null;
     return element;
   }
 
   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);
   }
 
   SourceString getParameterName(Send node) {
     var identifier = node.selector.asIdentifier();
     if (identifier !== null) {
       // Normal parameter: [:Type name:].
       return identifier.source;
     } else {
@@ skipping 99 matching lines @@
                                    MessageKind.EXTRA_FORMALS.error([]),
                                    api.Diagnostic.WARNING);
           }
         }
       }
       LinkBuilder<Element> parametersBuilder =
         visitor.analyzeNodes(formalParameters.nodes);
       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,
                                  visitor.optionalParameterCount,
                                  visitor.optionalParametersAreNamed,
                                  returnType);
   }
 
   // TODO(ahe): This is temporary.
   void resolveExpression(Node node) {
@@ skipping 258 matching lines @@
 
   Element localLookup(SourceString name) => library.find(name);
   Element lookup(SourceString name) => localLookup(name);
   Element lexicalLookup(SourceString name) => localLookup(name);
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
   String toString() => '$element';
 }