Fix warnings on Win64.

src/compiler.cc

Index: src/compiler.cc
diff --git a/src/compiler.cc b/src/compiler.cc
index 6be922ceedb3205da0e668941fe694e01ab5906a..eea2bc0d635828384da6cefc7e8acb48f33a1057 100644
--- a/src/compiler.cc
+++ b/src/compiler.cc
@@ -177,7 +177,8 @@ static Handle<JSFunction> MakeFunction(bool is_global,
     if (is_eval) {
       JavaScriptFrameIterator it;
       script->set_eval_from_function(it.frame()->function());
-      int offset = it.frame()->pc() - it.frame()->code()->instruction_start();
+      int offset = static_cast<int>(
+          it.frame()->pc() - it.frame()->code()->instruction_start());
       script->set_eval_from_instructions_offset(Smi::FromInt(offset));
     }
   }