Add script that generates X509 certificates for testing dart:io SecureSocket

tests/standalone/io/certificates/README

     This directory, tests/standalone/io/certificates, contains the
 X509 TLS certificates and private keys needed to run tests of Dart's
 secure networking code.  The SecureSocket and SecureServer classes
 are tested by making TLS (formerly called SSL) connections, secured
 by certificates from a self-signed test root authority.
 
+The certificates are created by running ../create_sample_certificates.sh
+in a bash or sh shell, with the openssl tools installed.  Run the script
+twice to create the untrusted_* files.
+
 server_chain.pem:
   Contains the chain of certificates, from the self-signed
 test certificate authority, through the intermediate CA, to the server
 certificate, used on the server side of a test connection.
 
 server_key.pem:
   Contains the private key for the server certificate
 
 trusted_certs.pem:
   Contains the self-signed certificate of the test certificate authority.
 This certificate is set as "trusted" by the client side of the connection
 in its SecurityContext object, so that a verified TLS connection to the
 server can be made.
 
 untrusted_server_chain.pem:
   Contains a chain of certificates, from a different self-signed
 test certificate authority, through an intermediate CA, to a server
 certificate, used on the server side of a test connection that is intended
 to fail because the client does not accept this certificate authority
 
 untrusted_server_key.pem:
   Contains the private key for the untrusted server certificate 
 in untrusted_server_chain.pem