make replacements work in e2e rooms

src/models/event.js

@@ -758,6 +758,10 @@ utils.extend(module.exports.MatrixEvent.prototype, {
         if (this.isRedacted()) {
             return;
         }
+        if (newEvent.isBeingDecrypted()) {
+            throw new Error("Trying to replace event when " +
+                "new content hasn't been decrypted yet");
+        }
         const oldContent = this.getContent();
         const newContent = newEvent.getContent()["m.new_content"];
         // need to always replace m.relates_to with the old one,

src/models/room.js

@@ -1038,8 +1038,16 @@ Room.prototype._addLiveEvent = function(event, duplicateStrategy) {
         const replacedId = relatesTo && relatesTo.event_id;
         const replacedEvent = this.getUnfilteredTimelineSet().findEventById(replacedId);
         if (replacedEvent) {
+            const doAndEmitReplacement = () => {
                 replacedEvent.makeReplaced(event);
                 this.emit("Room.replaceEvent", replacedEvent, this);
+            };
+
+            if (event.isBeingDecrypted()) {
+                event.once("Event.decrypted", doAndEmitReplacement);
+            } else {
+                doAndEmitReplacement();
+            }
         }
     }