Add pick file unit test

packages/image_picker/image_picker_for_web/lib/image_picker_for_web.dart

@@ -13,14 +13,15 @@ final String _kAcceptVideoMimeType = 'video/*';
 ///
 /// This class implements the `package:image_picker` functionality for the web.
 class ImagePickerPlugin extends ImagePickerPlatform {
+  final Function _overrideCreateInput;
+  bool get _shouldOverrideInput => _overrideCreateInput != null;
+
   html.Element _target;
 
-  /// A constructor that allows tests to override the window object used by the plugin.
-  ImagePickerPlugin({@visibleForTesting html.Element target})
-      : _target = target {
-    if (_target == null) {
-      _target = _initTarget(_kImagePickerInputsDomId);
-    }
+  /// A constructor that allows tests to override the function that creates file inputs.
+  ImagePickerPlugin({@visibleForTesting Function overrideCreateInput})
+      : _overrideCreateInput = overrideCreateInput {
+    _target = _initTarget(_kImagePickerInputsDomId);
   }
 
   /// Registers this class as the default instance of [ImagePickerPlatform].
@@ -122,6 +123,10 @@ class ImagePickerPlugin extends ImagePickerPlatform {
   html.Element _createInputElement(String accept, String capture) {
     html.Element element;
 
+    if (_shouldOverrideInput) {
+      return _overrideCreateInput(accept, capture);
+    }
+
     if (capture != null) {
       // Capture is not supported by dart:html :/
       element = html.Element.html(
@@ -137,8 +142,10 @@ class ImagePickerPlugin extends ImagePickerPlatform {
 
   /// Injects the file input element, and clicks on it
   void _injectAndActivate(html.Element element) {
-    _target.children.clear();
-    _target.children.add(element);
+    if (!_shouldOverrideInput) {
+      _target.children.clear();
+      _target.children.add(element);
+    }
     element.click();
   }
 }

packages/image_picker/image_picker_for_web/test/image_picker_for_web_test.dart

@@ -4,12 +4,74 @@
 
 @TestOn('chrome') // Uses web-only Flutter SDK
 
+import 'dart:async';
+import 'dart:convert';
 import 'dart:html' as html;
+import 'dart:typed_data';
+
 import 'package:flutter_test/flutter_test.dart';
+import 'package:image_picker_for_web/image_picker_for_web.dart';
+import 'package:image_picker_platform_interface/image_picker_platform_interface.dart';
 import 'package:mockito/mockito.dart';
 
-class MockWindow extends Mock implements html.Window {}
+final String expectedStringContents = "Hello, world!";
+final Uint8List bytes = utf8.encode(expectedStringContents);
+final html.File textFile = html.File([bytes], "hello.txt");
+
+class MockFileInput extends Mock implements html.FileUploadInputElement {}
+
+class MockOnChangeEvent extends Mock implements html.Event {
+  @override
+  MockFileInput target;
+}
+
+class MockElementStream<T extends html.Event> extends Mock
+    implements html.ElementStream<T> {
+  final StreamController<T> controller = StreamController<T>();
+  @override
+  StreamSubscription<T> listen(void onData(T event),
+      {Function onError, void onDone(), bool cancelOnError}) {
+    return controller.stream.listen(onData,
+        onError: onError, onDone: onDone, cancelOnError: cancelOnError);
+  }
+}
 
 void main() {
+  MockFileInput mockInput = MockFileInput();
+  MockElementStream mockStream = MockElementStream<html.Event>();
+  MockElementStream mockErrorStream = MockElementStream<html.Event>();
+  MockOnChangeEvent mockEvent = MockOnChangeEvent()..target = mockInput;
+
+  // Under test...
+  ImagePickerPlugin plugin =
+      ImagePickerPlugin(overrideCreateInput: (_, __) => mockInput);
+
+  setUp(() {
+    // Make the mockInput behave like a proper input...
+    when(mockInput.onChange).thenAnswer((_) => mockStream);
+    when(mockInput.onError).thenAnswer((_) => mockErrorStream);
+  });
+
+  tearDown(() {
+    reset(mockInput);
+  });
+
+  // Pick a file...
+  test('Can select a file, happy case', () async {
+    // Init the pick file dialog...
+    final file = plugin.pickImage(
+      source: ImageSource.gallery,
+    );
+
+    // Mock the browser behavior of selecting a file...
+    when(mockInput.files).thenReturn([textFile]);
+    mockStream.controller.add(mockEvent);
+
+    // Now the file should be selected
+    expect(file, completes);
+    // And readable
+    expect((await file).readAsString(), completion(expectedStringContents));
+  });
 
+  // Creates the correct DOM for the input...
 }