Fix regression in CheckstyleFormatter backwards compatibility

src/formatters/checkstyleFormatter.ts

@@ -38,8 +38,8 @@ export class Formatter extends AbstractFormatter {
     };
     /* tslint:enable:object-literal-sort-keys */
 
-    public format(failures: RuleFailure[], _fixes: RuleFailure[], fileNames: string[]): string {
-        const groupedFailures: { [k: string]: RuleFailure[] } = {};
+    public format(failures: RuleFailure[], _fixes?: RuleFailure[], fileNames?: string[]): string {
+        const groupedFailures: { [fileName: string]: RuleFailure[] } = {};
         for (const failure of failures) {
             const fileName = failure.getFileName();
             if (groupedFailures[fileName] !== undefined) {
@@ -49,6 +49,10 @@ export class Formatter extends AbstractFormatter {
             }
         }
 
+        if (fileNames === undefined) {
+            fileNames = Object.keys(groupedFailures);
+        }
+
         const formattedFiles = fileNames.map(fileName => {
             const formattedFailures =
                 groupedFailures[fileName] !== undefined

src/language/formatter/abstractFormatter.ts

@@ -23,8 +23,8 @@ export abstract class AbstractFormatter implements IFormatter {
     public static metadata: IFormatterMetadata;
     public abstract format(
         failures: RuleFailure[],
-        fixes: RuleFailure[],
-        fileNames: string[],
+        fixes?: RuleFailure[],
+        fileNames?: string[],
     ): string;
 
     protected sortFailures(failures: RuleFailure[]): RuleFailure[] {