Update harness getDirectories implementation for shims

Author: mhegazy

src/harness/harnessLanguageService.ts

@@ -274,7 +274,7 @@ namespace Harness.LanguageService {
         getCompilationSettings(): string { return JSON.stringify(this.nativeHost.getCompilationSettings()); }
         getCancellationToken(): ts.HostCancellationToken { return this.nativeHost.getCancellationToken(); }
         getCurrentDirectory(): string { return this.nativeHost.getCurrentDirectory(); }
-        getDirectories(path: string) { return this.nativeHost.getDirectories(path); }
+        getDirectories(path: string): string { return JSON.stringify(this.nativeHost.getDirectories(path)); }
         getDefaultLibFileName(): string { return this.nativeHost.getDefaultLibFileName(); }
         getScriptFileNames(): string { return JSON.stringify(this.nativeHost.getScriptFileNames()); }
         getScriptSnapshot(fileName: string): ts.ScriptSnapshotShim {