message: don't allow newline parts of longer than one newline
diff: @@ -57,7 +57,7 @@ class BasePart {
  appendUntilRejected(str) {
  for (let i = 0; i < str.length; ++i) {
  const chr = str.charAt(i);
  - if (!this.acceptsInsertion(chr)) {
  + if (!this.acceptsInsertion(chr, i)) {
  this._text = this._text + str.substr(0, i);
  return str.substr(i);
  }
  @@ -180,8 +180,8 @@ class PillPart extends BasePart {
  }
  export class NewlinePart extends BasePart {
  - acceptsInsertion(chr) {
  - return this.text.length === 0 && chr === "\n";
  + acceptsInsertion(chr, i) {
  + return (this.text.length + i) === 0 && chr === "\n";
  }
  acceptsRemoval(position, chr) {
  