Avoid ESLint warnings

Author: pbugnion

packages/controls/src/widget_upload.ts

@@ -72,7 +72,7 @@ export class FileUploadView extends DOMWidgetView {
         promisesFile.push(
           new Promise((resolve, reject) => {
             const fileReader = new FileReader();
-            fileReader.onload = () => {
+            fileReader.onload = (): void => {
               // We know we can read the result as an array buffer since
               // we use the `.readAsArrayBuffer` method
               const content: ArrayBuffer = fileReader.result as ArrayBuffer;
@@ -85,7 +85,7 @@ export class FileUploadView extends DOMWidgetView {
                 error: ''
               });
             };
-            fileReader.onerror = () => {
+            fileReader.onerror = (): void => {
               reject();
             };
             fileReader.onabort = fileReader.onerror;

packages/controls/test/src/widget_upload_test.ts

@@ -14,7 +14,7 @@ function getProxyButton(view: widgets.FileUploadView): HTMLButtonElement {
   return elem as HTMLButtonElement;
 }
 
-function fileInputForModel(model: widgets.FileUploadModel) {
+function fileInputForModel(model: widgets.FileUploadModel): HTMLInputElement {
   // For a given model, create and render a view and return the
   // view's input.
   const options = { model };
@@ -23,21 +23,23 @@ function fileInputForModel(model: widgets.FileUploadModel) {
   return getFileInput(view);
 }
 
-function proxyButtonForModel(model: widgets.FileUploadModel) {
+function proxyButtonForModel(
+  model: widgets.FileUploadModel
+): HTMLButtonElement {
   const options = { model };
   const view = new widgets.FileUploadView(options);
   view.render();
   return getProxyButton(view);
 }
 
-function simulateUpload(fileInput: HTMLInputElement, files: Array<File>) {
+function simulateUpload(fileInput: HTMLInputElement, files: Array<File>): void {
   // The 'files' property on an input element is normally not writeable
   // programmatically, so we explicitly overwrite it.
 
   // The type of fileInput.files is FileList, an Array with an
   // extra `.item` method.
   const fileList: any = files;
-  fileList.item = (index: number) => files[index];
+  fileList.item = (index: number): File => files[index];
   Object.defineProperty(fileInput, 'files', {
     value: fileList,
     writable: false