Cleanup handling of NullType in type propagation.

runtime/vm/object.cc

 // 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.
 
 #include "vm/object.h"
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/assembler.h"
 #include "vm/bigint_operations.h"
@@ skipping 1820 matching lines @@
   }
   // In the case of a subtype test, each occurrence of DynamicType in type S is
   // interpreted as the bottom type, a subtype of all types.
   // However, DynamicType is not more specific than any type.
   if (IsDynamicClass()) {
     return test_kind == kIsSubtypeOf;
   }
   // Check for NullType, which is not a subtype of any type, but is more
   // specific than any type.
   if (IsNullClass()) {
-    return test_kind == kIsMoreSpecificThan;
+    // User code cannot refer to class Null, therefore, we can only encounter
+    // NullType here as the type of the null constant, which must be treated
+    // separately in 'instance of' checks. Therefore, the NullType can only
+    // be encountered here during optimizations in 'more specific than' tests.
+    ASSERT(test_kind == kIsMoreSpecificThan);
+    return true;
   }
   // Check for reflexivity.
   if (raw() == other.raw()) {
     const intptr_t len = NumTypeArguments();
     if (len == 0) {
       return true;
     }
     // Since we do not truncate the type argument vector of a subclass (see
     // below), we only check a prefix of the proper length.
     // Check for covariance.
@@ skipping 9372 matching lines @@
 const char* JSRegExp::ToCString() const {
   const String& str = String::Handle(pattern());
   const char* format = "JSRegExp: pattern=%s flags=%s";
   intptr_t len = OS::SNPrint(NULL, 0, format, str.ToCString(), Flags());
   char* chars = Isolate::Current()->current_zone()->Alloc<char>(len + 1);
   OS::SNPrint(chars, (len + 1), format, str.ToCString(), Flags());
   return chars;
 }
 
 }  // namespace dart