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

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index 95109ae759d0d399ec0d084b6f43c0b437120f99..28253596e10a827faec62662f604744b75bb168b 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -92,6 +92,8 @@ class ElementKind {
     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;
 }
@@ -664,6 +666,16 @@ class SynthesizedConstructorElement extends FunctionElement {
   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;