Add VoidElement and VoidType to make sure that there is a canonical void type.

lib/compiler/implementation/elements/elements.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.
 
 #library('elements');
 
 #import('../tree/tree.dart');
 #import('../scanner/scannerlib.dart');
 #import('../leg.dart');  // TODO(karlklose): we only need type.
 #import('../util/util.dart');
@@ skipping 74 matching lines @@
     const ElementKind('library', ElementCategory.NONE);
   static final ElementKind PREFIX =
     const ElementKind('prefix', ElementCategory.PREFIX);
   static final ElementKind TYPEDEF =
     const ElementKind('typedef', ElementCategory.ALIAS);
 
   static final ElementKind STATEMENT =
     const ElementKind('statement', ElementCategory.NONE);
   static final ElementKind LABEL =
     const ElementKind('label', ElementCategory.NONE);
+  static final ElementKind VOID =
+    const ElementKind('void', ElementCategory.NONE);
 
   toString() => id;
 }
 
 class Element implements Hashable {
   final SourceString name;
   final ElementKind kind;
   final Element enclosingElement;
   Modifiers get modifiers() => null;
 
@@ skipping 552 matching lines @@
 }
 
 class SynthesizedConstructorElement extends FunctionElement {
   SynthesizedConstructorElement(Element enclosing)
     : super(enclosing.name, ElementKind.GENERATIVE_CONSTRUCTOR,
             null, enclosing);
 
   Token position() => enclosingElement.position();
 }
 
+class VoidElement extends Element {
+  VoidElement(Element enclosing)
+      : super(Types.VOID, ElementKind.VOID, enclosing);
+  Type computeType(compiler) => compiler.types.voidType;
+  Node parseNode(_) {
+    throw 'internal error: parseNode on void';
+  }
+  bool impliesType() => true;
+}
+
 class ClassElement extends ContainerElement {
   Type type;
   Type supertype;
   Type defaultClass;
   Link<Element> members = const EmptyLink<Element>();
   Map<SourceString, Element> localMembers;
   Map<SourceString, Element> constructors;
   Link<Type> interfaces = const EmptyLink<Type>();
   LinkedHashMap<SourceString, TypeVariableElement> typeParameters;
   bool isResolved = false;
@@ skipping 353 matching lines @@
   final Node node;
   Type bound;
   Type type;
   TypeVariableElement(name, Element enclosing, this.node, this.type,
                       [this.bound])
     : super(name, ElementKind.TYPE_VARIABLE, enclosing);
   Type computeType(compiler) => type;
   Node parseNode(compiler) => node;
   toString() => "${enclosingElement.toString()}.${name.slowToString()}";
 }