Comment 1:

Why? We left ECB out intentionally: it's insecure, and if needed it's
trivial to implement.

Comment 2 by hebipp1:

Well, I need it to implement a protocol (to encrypt the IV that's sended before the
actual content) and I don't know much about cryptography, so I'm a bit cautious to
implement it myself.

Comment 3:

IV are supposed to be public and shouldn't be encrypted, but anyway...
ECB is essentially "no mode" and while it isn't supported in a named fashion,
crypto/cipher.Block.Encrypt and Decrypt implement it on a block by block basis. You just
need to feed in a multiple of the blocksize:
bs := block.BlockSize()
if len(plaintext) % bs != 0 {
  panic("Need a multiple of the blocksize")
}
ciphertext := make([]byte, len(plaintext))
for len(plaintext) > 0 {
  block.Encrypt(ciphertext, plaintext)
  plaintext = plaintext[bs:]
  ciphertext = ciphertext[bs:]
}
[1] http://golang.org/pkg/crypto/cipher/#Block

Status changed to WontFix.