VM: Use read-only handle Object::dynamic_type() where possible.

runtime/vm/ast_transformer.cc

Index: runtime/vm/ast_transformer.cc
diff --git a/runtime/vm/ast_transformer.cc b/runtime/vm/ast_transformer.cc
index 95d8d4f37dc8ce33dab80a2117276d9731a2f5dc..ae42b7089427b044851036e7351046b8e8b97a2a 100644
--- a/runtime/vm/ast_transformer.cc
+++ b/runtime/vm/ast_transformer.cc
@@ -71,7 +71,7 @@ LocalVariable* AwaitTransformer::EnsureCurrentTempVar() {
   if (await_tmp == NULL) {
     // We need a new temp variable; add it to the function's top scope.
     await_tmp = new (Z) LocalVariable(
-        Scanner::kNoSourcePos, symbol, Type::ZoneHandle(Type::DynamicType()));
+        Scanner::kNoSourcePos, symbol, Object::dynamic_type());
     async_temp_scope_->AddVariable(await_tmp);
     // After adding it to the top scope, we can look it up from the preamble.
     // The following call includes an ASSERT check.