src/vs/editor/common/model/mirrorTextModel.ts/MirrorTextModel/_setLineText
class MirrorTextModel:	private _setLineText(lineIndex: number, newValue: string): void {
		this._lines[lineIndex] = newValue;
		if (this._lineStarts) {
			// update prefix sum
			this._lineStarts.setValue(lineIndex, this._lines[lineIndex].length + this._eol.length);
		}
	}