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