Move Null type to the Bottom in the VM (fixes #28025).

runtime/vm/precompiler.cc

 // Copyright (c) 2015, 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/precompiler.h"
 
 #include "vm/aot_optimizer.h"
 #include "vm/assembler.h"
 #include "vm/ast_printer.h"
 #include "vm/branch_optimizer.h"
@@ skipping 35 matching lines @@
 #include "vm/version.h"
 #include "vm/json_parser.h"
 
 namespace dart {
 
 #define T (thread())
 #define I (isolate())
 #define Z (zone())
 
 
-DEFINE_FLAG(bool, print_unique_targets, false, "Print unique dynaic targets");
+DEFINE_FLAG(bool, print_unique_targets, false, "Print unique dynamic targets");
 DEFINE_FLAG(bool, trace_precompiler, false, "Trace precompiler.");
 DEFINE_FLAG(
     int,
     max_speculative_inlining_attempts,
     1,
     "Max number of attempts with speculative inlining (precompilation only)");
 DEFINE_FLAG(int, precompiler_rounds, 1, "Number of precompiler iterations");
 
 DECLARE_FLAG(bool, allocation_sinking);
 DECLARE_FLAG(bool, common_subexpression_elimination);
@@ skipping 200 matching lines @@
   *upper_limit = -1;
   intptr_t last_matching_cid = -1;
 
   ClassTable* table = thread_->isolate()->class_table();
   Class& cls = Class::Handle(zone);
   AbstractType& cls_type = AbstractType::Handle(zone);
   for (intptr_t cid = kInstanceCid; cid < table->NumCids(); cid++) {
     if (!table->HasValidClassAt(cid)) continue;
     if (cid == kVoidCid) continue;
     if (cid == kDynamicCid) continue;
+    if (cid == kNullCid) continue;  // Instance is not at Bottom like Null type.
     cls = table->At(cid);
     if (cls.is_abstract()) continue;
     if (cls.is_patch()) continue;
     if (cls.IsTopLevel()) continue;
 
     cls_type = cls.RareType();
     if (cls_type.IsSubtypeOf(type, NULL, NULL, Heap::kOld)) {
       last_matching_cid = cid;
       if (*lower_limit == -1) {
         // Found beginning of range.
@@ skipping 3368 matching lines @@
 
   ASSERT(FLAG_precompiled_mode);
   const bool optimized = function.IsOptimizable();  // False for natives.
   DartPrecompilationPipeline pipeline(zone, field_type_map);
   return PrecompileFunctionHelper(precompiler, &pipeline, function, optimized);
 }
 
 #endif  // DART_PRECOMPILER
 
 }  // namespace dart