Support check mode for statics.

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 const bool LOG_FAILURES = false;
 
@@ skipping 235 matching lines @@
              Element dynamicElement,
              LibraryElement library)
     : voidType = new VoidType(new VoidElement(library)),
       dynamicType = new InterfaceType(dynamicElement);
 
   /** Returns true if t is a subtype of s */
   bool isSubtype(DartType t, DartType s) {
     if (t === s ||
         t === dynamicType ||
         s === dynamicType ||
-        s.element === compiler.objectClass) {
+        s.element === compiler.objectClass ||
+        t.element === compiler.nullClass) {
       return true;
     }
     t = t.unalias(compiler);
     s = s.unalias(compiler);
 
     if (t is VoidType) {
       return false;
     } else if (t is InterfaceType) {
       if (s is !InterfaceType) return false;
       ClassElement tc = t.element;
@@ skipping 636 matching lines @@
   }
 
   visitCatchBlock(CatchBlock node) {
     return unhandledStatement();
   }
 
   visitTypedef(Typedef node) {
     return unhandledStatement();
   }
 }