Follow up on #18424 comments (#20125)

packages/flutter/lib/src/painting/image_stream.dart

@@ -151,8 +151,6 @@ class ImageStream extends Diagnosticable {
   /// Many `listener`s can have the same `onError` and one `listener` can also
   /// have multiple `onError` by invoking [addListener] multiple times with
   /// a different `onError` each time.
-  ///
-  /// Repeated `onError` will only be called once when an error occurs.
   void addListener(ImageListener listener, { ImageErrorListener onError }) {
     if (_completer != null)
       return _completer.addListener(listener, onError: onError);
@@ -166,9 +164,12 @@ class ImageStream extends Diagnosticable {
     if (_completer != null)
       return _completer.removeListener(listener);
     assert(_listeners != null);
-    _listeners.removeWhere((_ImageListenerPair listenerPair) {
-      return listenerPair.listener == listener;
-    });
+    for (int i = 0; i < _listeners.length; ++i) {
+      if (_listeners[i].listener == listener) {
+        _listeners.removeAt(i);
+        continue;
+      }
+    }
   }
 
   /// Returns an object which can be used with `==` to determine if this
@@ -242,26 +243,29 @@ abstract class ImageStreamCompleter extends Diagnosticable {
     }
     if (_currentError != null && onError != null) {
       try {
-          onError(_currentError.exception, _currentError.stack);
-        } catch (exception, stack) {
-          FlutterError.reportError(
-            new FlutterErrorDetails(
-              exception: exception,
-              library: 'image resource service',
-              context: 'by a synchronously-called image error listener',
-              stack: stack,
-            ),
-          );
-        }
+        onError(_currentError.exception, _currentError.stack);
+      } catch (exception, stack) {
+        FlutterError.reportError(
+          new FlutterErrorDetails(
+            exception: exception,
+            library: 'image resource service',
+            context: 'by a synchronously-called image error listener',
+            stack: stack,
+          ),
+        );
+      }
     }
   }
 
   /// Stop listening for new concrete [ImageInfo] objects and errors from
   /// its associated [ImageErrorListener].
   void removeListener(ImageListener listener) {
-    _listeners.removeWhere((_ImageListenerPair listenerPair) {
-      return listenerPair.listener == listener;
-    });
+    for (int i = 0; i < _listeners.length; ++i) {
+      if (_listeners[i].listener == listener) {
+        _listeners.removeAt(i);
+        continue;
+      }
+    }
   }
 
   /// Calls all the registered listeners to notify them of a new image.
@@ -289,9 +293,6 @@ abstract class ImageStreamCompleter extends Diagnosticable {
   /// Calls all the registered error listeners to notify them of an error that
   /// occurred while resolving the image.
   ///
-  /// If the same error listener is attached with multiple listeners, that
-  /// error listener will only be notified once.
-  ///
   /// If no error listeners are attached, a [FlutterError] will be reported
   /// instead.
   @protected
@@ -311,7 +312,6 @@ abstract class ImageStreamCompleter extends Diagnosticable {
       silent: silent,
     );
 
-    // Many listeners can have the same error listener. De-duplicate.
     final List<ImageErrorListener> localErrorListeners =
         _listeners.map<ImageErrorListener>(
           (_ImageListenerPair listenerPair) => listenerPair.errorListener

packages/flutter/test/widgets/image_test.dart

@@ -541,21 +541,15 @@ void main() {
     expect(capturedImage, isNull); // The image stream listeners should never be called.
   });
 
-  testWidgets('Removing duplicate listeners removes error listeners', (WidgetTester tester) async {
-    bool errorListenerCalled = false;
-    dynamic reportedException;
-    StackTrace reportedStackTrace;
+  testWidgets('Removing listener removes one listener and error listener', (WidgetTester tester) async {
+    int errorListenerCalled = 0;
     ImageInfo capturedImage;
     final ImageErrorListener errorListener = (dynamic exception, StackTrace stackTrace) {
-      errorListenerCalled = true;
+      errorListenerCalled++;
     };
     final ImageListener listener = (ImageInfo info, bool synchronous) {
       capturedImage = info;
     };
-    FlutterError.onError = (FlutterErrorDetails flutterError) {
-      reportedException = flutterError.exception;
-      reportedStackTrace = flutterError.stack;
-    };
 
     final Exception testException = new Exception('cannot resolve host');
     final StackTrace testStack = StackTrace.current;
@@ -563,7 +557,7 @@ void main() {
     imageProvider._streamCompleter.addListener(listener, onError: errorListener);
     // Duplicates the same set of listener and errorListener.
     imageProvider._streamCompleter.addListener(listener, onError: errorListener);
-    // Now remove all specified listeners and associated error listeners.
+    // Now remove one entry of the specified listener and associated error listener.
     // Don't explicitly remove the error listener.
     imageProvider._streamCompleter.removeListener(listener);
     ImageConfiguration configuration;
@@ -583,10 +577,7 @@ void main() {
     await tester.idle(); // Let the failed completer's future hit the stream completer.
     expect(tester.binding.microtaskCount, 0);
 
-    expect(errorListenerCalled, false);
-    // Since the error listener is removed, bubble up to FlutterError.
-    expect(reportedException, testException);
-    expect(reportedStackTrace, testStack);
+    expect(errorListenerCalled, 1);
     expect(capturedImage, isNull); // The image stream listeners should never be called.
   });