Cleanup handling of NullType in type propagation.

runtime/vm/flow_graph_builder.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/flow_graph_builder.h"
 
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/code_descriptors.h"
 #include "vm/dart_entry.h"
@@ skipping 427 matching lines @@
   // If nothing is known about the value, as is the case for passed-in
   // parameters, and since dst_type is not one of the tested cases above, then
   // the type test cannot be eliminated.
   if (value == NULL) {
     return false;
   }
 
   // Propagated types are not set yet.
   // More checks will possibly be eliminated during type propagation.
   const bool eliminated = value->CompileTypeIsMoreSpecificThan(dst_type);
+  // Note: 'eliminated' is true for a null constant value, since its type,
+  // i.e. bottom type, is more specific than any type.
   if (FLAG_trace_type_check_elimination) {
     FlowGraphPrinter::PrintTypeCheck(owner()->parsed_function(),
                                      token_pos,
                                      value,
                                      dst_type,
                                      dst_name,
                                      eliminated);
   }
   return eliminated;
 }
@@ skipping 2389 matching lines @@
   intptr_t len = OS::SNPrint(NULL, 0, kFormat, function_name, reason) + 1;
   char* chars = Isolate::Current()->current_zone()->Alloc<char>(len);
   OS::SNPrint(chars, len, kFormat, function_name, reason);
   const Error& error = Error::Handle(
       LanguageError::New(String::Handle(String::New(chars))));
   Isolate::Current()->long_jump_base()->Jump(1, error);
 }
 
 
 }  // namespace dart