use .get fallback where .getKey is not supported (Edge 15)

src/matrix/storage/idb/query-target.js

@@ -17,6 +17,10 @@ export default class QueryTarget {
         }
     }
 
+    supports(methodName) {
+        return this._target.supports(methodName);
+    }
+
     get(key) {
         return reqAsPromise(this._target.get(key));
     }

src/matrix/storage/idb/store.js

@@ -7,6 +7,10 @@ class QueryTargetWrapper {
         this._qt = qt;
     }
 
+    supports(methodName) {
+        return !!this._qt[methodName];
+    }
+    
     openKeyCursor(...params) {
         try {
             return this._qt.openKeyCursor(...params);

src/matrix/storage/idb/stores/PendingEventStore.js

@@ -36,7 +36,13 @@ export default class PendingEventStore {
 
     async exists(roomId, queueIndex) {
         const keyRange = IDBKeyRange.only(encodeKey(roomId, queueIndex));
-        const key = await this._eventStore.getKey(keyRange);
+        let key;
+        if (this._eventStore.supports("getKey")) {
+            key = await this._eventStore.getKey(keyRange);
+        } else {
+            const value = await this._eventStore.get(keyRange);
+            key = value && value.key;
+        }
         return !!key;
     }