VM: Fix incorrect canonicalization rule for `UnboxedIntConverterInstr`.

runtime/vm/intermediate_language.cc

 // Copyright (c) 2013, 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/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/bootstrap.h"
 #include "vm/compiler.h"
 #include "vm/constant_propagator.h"
@@ skipping 2292 matching lines @@
 }
 
 
 Definition* UnboxedIntConverterInstr::Canonicalize(FlowGraph* flow_graph) {
   if (!HasUses()) return NULL;
 
   UnboxedIntConverterInstr* box_defn =
       value()->definition()->AsUnboxedIntConverter();
   if ((box_defn != NULL) && (box_defn->representation() == from())) {
     if (box_defn->from() == to()) {
+      // Do not erase truncating convertions from 64-bit value to 32-bit values
+      // because such convertions erase upper 32 bits.
+      if ((box_defn->from() == kUnboxedMint) && box_defn->is_truncating()) {
+        return this;
+      }
       return box_defn->value()->definition();
     }
 
     UnboxedIntConverterInstr* converter = new UnboxedIntConverterInstr(
         box_defn->from(), representation(), box_defn->value()->CopyWithType(),
         (to() == kUnboxedInt32) ? GetDeoptId() : Thread::kNoDeoptId);
     if ((representation() == kUnboxedInt32) && is_truncating()) {
       converter->mark_truncating();
     }
     flow_graph->InsertBefore(this, converter, env(), FlowGraph::kValue);
@@ skipping 1663 matching lines @@
   set_native_c_function(native_function);
   function().SetIsNativeAutoSetupScope(auto_setup_scope);
   Dart_NativeEntryResolver resolver = library.native_entry_resolver();
   bool is_bootstrap_native = Bootstrap::IsBootstapResolver(resolver);
   set_is_bootstrap_native(is_bootstrap_native);
 }
 
 #undef __
 
 }  // namespace dart