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

lib/compiler/implementation/typechecker.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.
 
 class TypeCheckerTask extends CompilerTask {
   TypeCheckerTask(Compiler compiler) : super(compiler);
   String get name() => "Type checker";
 
   static final bool LOG_FAILURES = false;
 
@@ skipping 42 matching lines @@
   static final MAYBE_RETURNING = const StatementType('<maybe returning>');
 
   /** Combine the information about two control-flow edges that are joined. */
   StatementType join(StatementType other) {
     return (this === other) ? this : MAYBE_RETURNING;
   }
 
   String toString() => stringName;
 }
 
+class VoidType implements Type {
+  const VoidType(this.element);
+  SourceString get name() => Types.VOID;
+  final VoidElement element;
+}
+
 class InterfaceType implements Type {
   final Element element;
   final Link<Type> arguments;
 
   const InterfaceType(this.element,
                       [this.arguments = const EmptyLink<Type>()]);
 
   SourceString get name() => element.name;
 
   toString() {
@@ skipping 31 matching lines @@
 class Types {
   static final VOID = const SourceString('void');
   static final INT = const SourceString('int');
   static final DOUBLE = const SourceString('double');
   static final DYNAMIC = const SourceString('Dynamic');
   static final STRING = const SourceString('String');
   static final BOOL = const SourceString('bool');
   static final OBJECT = const SourceString('Object');
   static final LIST = const SourceString('List');
 
-  final InterfaceType voidType;
+  final VoidType voidType;
   final InterfaceType dynamicType;
 
   Types(Element dynamicElement)
     : this.with(dynamicElement, new LibraryElement(new Script(null, null)));
 
   // TODO(karlklose): should we have a class Void?
   Types.with(Element dynamicElement, LibraryElement library)
-    : voidType = new InterfaceType(new ClassElement(VOID, library)),
+    : voidType = new VoidType(new VoidElement(library)),
       dynamicType = new InterfaceType(dynamicElement);
 
   Type lookup(SourceString s) {
     if (VOID == s) {
       return voidType;
     } else if (DYNAMIC == s || s.stringValue === 'var') {
       return dynamicType;
     }
     return null;
   }
 
   /** Returns true if t is a subtype of s */
   bool isSubtype(Type t, Type s) {
     if (t === s || t === dynamicType || s === dynamicType ||
         s.name == OBJECT) return true;
-    if (t is InterfaceType) {
+    if (t is VoidType) {
+      return false;
+    } else if (t is InterfaceType) {
       if (s is !InterfaceType) return false;
       ClassElement tc = t.element;
       if (tc === s.element) return true;
       for (Link<Type> supertypes = tc.allSupertypes;
            supertypes != null && !supertypes.isEmpty();
            supertypes = supertypes.tail) {
         Type supertype = supertypes.head;
         if (supertype.element === s.element) return true;
       }
       return false;
@@ skipping 422 matching lines @@
     return type;
   }
 
   Type visitOperator(Operator node) {
     fail(node, 'internal error');
   }
 
   /** Dart Programming Language Specification: 11.10 Return */
   Type visitReturn(Return node) {
     final expression = node.expression;
-    final isVoidFunction =
-        (expectedReturnType.element === compiler.types.voidType.element);
+    final isVoidFunction = (expectedReturnType === types.voidType);
 
     // Executing a return statement return e; [...] It is a static type warning
     // if the type of e may not be assigned to the declared return type of the
     // immediately enclosing function.
     if (expression !== null) {
       final expressionType = analyze(expression);
       if (isVoidFunction
           && !types.isAssignable(expressionType, types.voidType)) {
         reportTypeWarning(expression, MessageKind.RETURN_VALUE_IN_VOID,
                           [expressionType]);
@@ skipping 136 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     fail(node);
   }
 
   visitTypedef(Typedef node) {
     fail(node);
   }
 }