Add snapshot size to the VM benchmarks suite.

runtime/vm/unit_test.cc

Index: runtime/vm/unit_test.cc
===================================================================
--- runtime/vm/unit_test.cc	(revision 18232)
+++ runtime/vm/unit_test.cc	(working copy)
@@ -86,6 +86,8 @@
       return Builtin::LoadAndCheckLibrary(Builtin::kUtfLibrary);
     } else if (DartUtils::IsDartCryptoLibURL(url_chars)) {
       return Builtin::LoadAndCheckLibrary(Builtin::kCryptoLibrary);
+    } else if (DartUtils::IsDartBuiltinLibURL(url_chars)) {
+      return Builtin::LoadAndCheckLibrary(Builtin::kBuiltinLibrary);
     } else {
       return Dart_Error("Do not know how to load '%s'", url_chars);
     }