src/vs/workbench/api/common/extHostDocumentData.ts/ExtHostDocumentData/_offsetAt
class ExtHostDocumentData:	private _offsetAt(position: vscode.Position): number {
		position = this._validatePosition(position);
		this._ensureLineStarts();
		return this._lineStarts!.getPrefixSum(position.line - 1) + position.character;
	}