src/vs/editor/common/diff/smartLinesDiffComputer.ts/CharSequence/getStartLineNumber
class CharSequence:	public getStartLineNumber(i: number): number {
		if (i > 0 && i === this._lineNumbers.length) {
			// the start line number of the element after the last element
			// is the end line number of the last element
			return this.getEndLineNumber(i - 1);
		}
		this._assertIndex(i, this._lineNumbers);

		return this._lineNumbers[i];
	}