How to use the tsickle.mergeEmitResults function in tsickle

program, bazelHost, bazelHost, compilerOpts, targetSourceFile, writeFile,
          cancellationToken, emitOnlyDtsFiles, {
            beforeTs: customTransformers.before,
            afterTs: customTransformers.after,
            afterDeclarations: customTransformers.afterDeclarations,
          });

  if (!gatherDiagnostics) {
    gatherDiagnostics = (program) =>
        gatherDiagnosticsForInputsOnly(compilerOpts, bazelOpts, program);
  }
  const {diagnostics, emitResult, program} = ng.performCompilation({
    rootNames: files,
    options: compilerOpts,
    host: ngHost, emitCallback,
    mergeEmitResultsCallback: tsickle.mergeEmitResults, gatherDiagnostics
  });
  const tsickleEmitResult = emitResult as tsickle.EmitResult;
  let externs = '/** @externs */\n';
  if (!diagnostics.length) {
    if (bazelOpts.tsickleGenerateExterns) {
      externs += tsickle.getGeneratedExterns(tsickleEmitResult.externs);
    }
    if (bazelOpts.manifest) {
      const manifest = constructManifest(tsickleEmitResult.modulesManifest, bazelHost);
      fs.writeFileSync(bazelOpts.manifest, manifest);
    }
  }

  // If compilation fails unexpectedly, performCompilation returns no program.
  // Make sure not to crash but report the diagnostics.
  if (!program) return {program, diagnostics};