src/vs/platform/configuration/common/configurationModels.ts/Configuration/allKeys
class Configuration:	allKeys(): string[] {
		const keys: Set<string> = new Set<string>();
		this._defaultConfiguration.freeze().keys.forEach(key => keys.add(key));
		this.userConfiguration.freeze().keys.forEach(key => keys.add(key));
		this._workspaceConfiguration.freeze().keys.forEach(key => keys.add(key));
		this._folderConfigurations.forEach(folderConfiguraiton => folderConfiguraiton.freeze().keys.forEach(key => keys.add(key)));
		return [...keys.values()];
	}