Do units survive if you use a reaction (like Janna) to heal a unit to its maximum health after it has taken damage but before the Imperial Decree trigger resolves, the unit will survive?

To answer your question: No, the unit will not survive.

Healing a unit does not prevent it from being killed by the effect of Imperial Decree.

Reasoning

According to the ruling in FAQ #8633, which clarifies how Imperial Decree functions:

• Imperial Decree creates a triggered ability that lasts for the remainder of the turn.
• The trigger fires whenever a unit "takes damage."
• "Takes damage" refers to the event of receiving damage, not the current state of being damaged.
• Healing a unit removes existing damage (Rule 405.1), but it does not "undo" the event of having taken damage earlier in the turn.

Sequence of Events

1. Damage Event: A unit takes damage. This triggers the Imperial Decree effect.
2. Chain: The Imperial Decree trigger is placed on the chain as a pending item.
3. Reaction: You play Janna as a reaction to the Imperial Decree trigger.
4. Resolution:
   • Janna resolves: Your units are healed.
   • Imperial Decree trigger resolves: The unit is killed.

Even though the unit is healed to its maximum health, the Imperial Decree trigger is still active and waiting to resolve. Because the unit already "took damage" earlier in the turn, the condition for the trigger is satisfied, and the unit will be killed when the trigger resolves.

Note: I could not fully verify this ruling against the retrieved rules.