src/vs/editor/common/model/mirrorTextModel.ts/MirrorTextModel/_ensureLineStarts
class MirrorTextModel:	protected _ensureLineStarts(): void {
		if (!this._lineStarts) {
			const eolLength = this._eol.length;
			const linesLength = this._lines.length;
			const lineStartValues = new Uint32Array(linesLength);
			for (let i = 0; i < linesLength; i++) {
				lineStartValues[i] = this._lines[i].length + eolLength;
			}
			this._lineStarts = new PrefixSumComputer(lineStartValues);
		}
	}