duplicate slate code where we changed it drastically

to still make it work when the feature flag will be turned off

src/SlateComposerHistoryManager.js

@@ -0,0 +1,86 @@
+//@flow
+/*
+Copyright 2017 Aviral Dasgupta
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import {Value} from 'slate';
+
+import _clamp from 'lodash/clamp';
+
+type MessageFormat = 'rich' | 'markdown';
+
+class HistoryItem {
+    // We store history items in their native format to ensure history is accurate
+    // and then convert them if our RTE has subsequently changed format.
+    value: Value;
+    format: MessageFormat = 'rich';
+
+    constructor(value: ?Value, format: ?MessageFormat) {
+        this.value = value;
+        this.format = format;
+    }
+
+    static fromJSON(obj: Object): HistoryItem {
+        return new HistoryItem(
+            Value.fromJSON(obj.value),
+            obj.format,
+        );
+    }
+
+    toJSON(): Object {
+        return {
+            value: this.value.toJSON(),
+            format: this.format,
+        };
+    }
+}
+
+export default class SlateComposerHistoryManager {
+    history: Array<HistoryItem> = [];
+    prefix: string;
+    lastIndex: number = 0; // used for indexing the storage
+    currentIndex: number = 0; // used for indexing the loaded validated history Array
+
+    constructor(roomId: string, prefix: string = 'mx_composer_history_') {
+        this.prefix = prefix + roomId;
+
+        // TODO: Performance issues?
+        let item;
+        for (; item = sessionStorage.getItem(`${this.prefix}[${this.currentIndex}]`); this.currentIndex++) {
+            try {
+                this.history.push(
+                    HistoryItem.fromJSON(JSON.parse(item)),
+                );
+            } catch (e) {
+                console.warn("Throwing away unserialisable history", e);
+            }
+        }
+        this.lastIndex = this.currentIndex;
+        // reset currentIndex to account for any unserialisable history
+        this.currentIndex = this.history.length;
+    }
+
+    save(value: Value, format: MessageFormat) {
+        const item = new HistoryItem(value, format);
+        this.history.push(item);
+        this.currentIndex = this.history.length;
+        sessionStorage.setItem(`${this.prefix}[${this.lastIndex++}]`, JSON.stringify(item.toJSON()));
+    }
+
+    getItem(offset: number): ?HistoryItem {
+        this.currentIndex = _clamp(this.currentIndex + offset, 0, this.history.length - 1);
+        return this.history[this.currentIndex];
+    }
+}