Add Null class to dart:core.

runtime/vm/flow_graph_builder.cc

Index: runtime/vm/flow_graph_builder.cc
diff --git a/runtime/vm/flow_graph_builder.cc b/runtime/vm/flow_graph_builder.cc
index f9c395cd0c2c224f935a90ec88a342ab2295b797..fc39e60a527469ec36bf2b741edc437de63521fd 100644
--- a/runtime/vm/flow_graph_builder.cc
+++ b/runtime/vm/flow_graph_builder.cc
@@ -1176,11 +1176,12 @@ void ValueGraphVisitor::BuildTypeTest(ComparisonNode* node) {
     const Class& cls = Class::Handle(literal_value.clazz());
     ConstantInstr* result = NULL;
     if (cls.IsNullClass()) {
-      // A null object is only an instance of Object and dynamic, which has
-      // already been checked above (if the type is instantiated). So we can
-      // return false here if the instance is null (and if the type is
+      // A null object is only an instance of Object, dynamic and Null, the
+      // first two of which has already been checked above (if the type is
       // instantiated).
-      result = new ConstantInstr(negate_result ? Bool::True() : Bool::False());
+      const bool is_type = type.IsNullType();
+      result = new ConstantInstr((is_type != negate_result) ? Bool::True()
+                                                            : Bool::False());
     } else {
       Error& malformed_error = Error::Handle();
       if (literal_value.IsInstanceOf(type,