Fix some more shorten-64-to-32 warnings:

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/cpu.h"
@@ skipping 17976 matching lines @@
     return Smi::New(static_cast<intptr_t>(value));
   }
   return Mint::NewCanonical(value);
 }
 
 
 RawInteger* Integer::New(int64_t value, Heap::Space space, const bool silent) {
   const bool is_smi = Smi::IsValid(value);
   if (!silent &&
       FLAG_throw_on_javascript_int_overflow &&
-      !Utils::IsJavascriptInt64(value)) {
+      !Utils::IsJavascriptInt(value)) {
     const Integer& i = is_smi ?
         Integer::Handle(Smi::New(static_cast<intptr_t>(value))) :
         Integer::Handle(Mint::New(value, space));
     ThrowJavascriptIntegerOverflow(i);
   }
   if (is_smi) {
     return Smi::New(static_cast<intptr_t>(value));
   }
   return Mint::New(value, space);
 }
@@ skipping 77 matching lines @@
     value = AsInt64Value();
   } else if (IsMint()) {
     Mint& mint = Mint::Handle();
     mint ^= raw();
     value = mint.value();
   } else {
     if (Bigint::Cast(*this).FitsIntoInt64()) {
       value = AsInt64Value();
     }
   }
-  return !Utils::IsJavascriptInt64(value);
+  return !Utils::IsJavascriptInt(value);
 }
 
 
 RawInteger* Integer::AsValidInteger() const {
   if (FLAG_throw_on_javascript_int_overflow &&
       CheckJavascriptIntegerOverflow()) {
     ThrowJavascriptIntegerOverflow(*this);
   }
   if (IsSmi()) return raw();
   if (IsMint()) {
     Mint& mint = Mint::Handle();
     mint ^= raw();
     if (Smi::IsValid(mint.value())) {
       return Smi::New(static_cast<intptr_t>(mint.value()));
     } else {
       return raw();
     }
   }
   if (Bigint::Cast(*this).FitsIntoInt64()) {
     const int64_t value = AsInt64Value();
     if (Smi::IsValid(value)) {
-      return Smi::New(value);
+      // This cast is safe because Smi::IsValid verifies that value will fit.
+      intptr_t val = static_cast<intptr_t>(value);
+      return Smi::New(val);
     }
     return Mint::New(value);
   }
   return raw();
 }
 
 
 RawInteger* Integer::ArithmeticOp(Token::Kind operation,
                                   const Integer& other,
                                   Heap::Space space) const {
@@ skipping 4735 matching lines @@
   return tag_label.ToCString();
 }
 
 
 void UserTag::PrintJSONImpl(JSONStream* stream, bool ref) const {
   Instance::PrintJSONImpl(stream, ref);
 }
 
 
 }  // namespace dart