message: Fix arrows keys moving through edit history
  Different fix that fixes
  by setting the flag before the callback rather than having the update
  method set the flag.
  Regressed in
  Fixes
diff: @@ -126,7 +126,6 @@ export default class BasicMessageEditor extends React.Component {
  }
  _updateEditorState = (selection, inputType, diff) => {
  - this._modifiedFlag = true;
  renderModel(this._editorRef, this.props.model);
  if (selection) { // set the caret/selection
  try {
  @@ -212,6 +211,7 @@ export default class BasicMessageEditor extends React.Component {
  if (type === "cut") {
  // Remove the text, updating the model as appropriate
  replaceRangeAndMoveCaret(range, []);
  + this._modifiedFlag = true;
  }
  event.preventDefault();
  }
  @@ -238,6 +238,7 @@ export default class BasicMessageEditor extends React.Component {
  const text = event.clipboardData.getData("text/plain");
  parts = parsePlainTextMessage(text, partCreator);
  }
  + this._modifiedFlag = true;
  const range = getRangeForSelection(this._editorRef, model, document.getSelection());
  replaceRangeAndMoveCaret(range, parts);
  event.preventDefault();
  @@ -248,6 +249,7 @@ export default class BasicMessageEditor extends React.Component {
  if (this._isIMEComposing) {
  return;
  }
  + this._modifiedFlag = true;
  const sel = document.getSelection();
  const {caret, text} = getCaretOffsetAndText(this._editorRef, sel);
  this.props.model.update(text, event.inputType, caret);
  @@ -258,6 +260,7 @@ export default class BasicMessageEditor extends React.Component {
  const {caret, text} = getCaretOffsetAndText(this._editorRef, sel);
  const newText = text.substr(0, caret.offset) + textToInsert + text.substr(caret.offset);
  caret.offset += textToInsert.length;
  + this._modifiedFlag = true;
  this.props.model.update(newText, inputType, caret);
  }
  