Add built-in root certificates to dart:io SecureSocket.

tests/standalone/io/pkcert_empty/README

Index: tests/standalone/io/pkcert_empty/README
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+This is an empty certificate database used by Dart for testing purposes.
+
+It is created as a certificate database by NSS (Network Security Services),
+a library from Mozilla, using the certutil tool.  It uses a cert9.db file,
+rather than a cert8.db file, so the database directory must be specified with
+"sql:" in front of the directory path, or the environment variable
+NSS_DEFAULT_DB_TYPE must be set to "sql".
+
+The password for the key database is "".
+
+The database contains no certificates or keys.