Amber Heard: You Watch ... Johnny's Gonna Take the 5th

Johnny Depp will NEVER testify Friday in the restraining order trial ... he'll take the 5th because he's afraid he'll say something that could get him prosecuted for domestic violence -- that's the take of the sources connected to Amber Heard's case, but it may be a little unfair.

Our sources tell TMZ ... Amber's team is convinced Johnny is afraid to testify and, given the police are already involved, it's just too risky to take the oath and endure a hard cross-examination.

What's more ... Amber's team doesn't even believe Johnny will submit a written declaration denying domestic violence. If you recall, Mel Gibson submitted a written declaration in his custody war with Oksana Grigorieva, and in the doc he admitted he had slapped her. That's what got him prosecuted.

Same with Cosby. What he said in his depo formed the basis of the current prosecution.

Amber's team believes they have the restraining order in the bank. Their thinking goes this way ... Amber's eager to testify, and if Johnny is mum, the judge has no choice but to grant the restraining order.

Fact is ... Amber triggered a police investigation, and it's common practice for someone who's in Johnny's position to take the 5th in a civil case ... even if there's hard proof he's innocent.