src/vs/editor/common/services/editorSimpleWorker.ts/MirrorModel/offsetAt
class MirrorModel:	public offsetAt(position: IPosition): number {
		position = this._validatePosition(position);
		this._ensureLineStarts();
		return this._lineStarts!.getPrefixSum(position.lineNumber - 2) + (position.column - 1);
	}