Rationalise all our exception catching and reporting code

- make them all have the same style
- make them all include the stack trace last
- make them all stop printing if their callback is set (if
  appropriate, they don't yet all have callbacks)

packages/flutter/lib/src/gestures/binding.dart

@@ -82,8 +82,8 @@ abstract class Gesturer extends BindingBase implements HitTestTarget, HitTestabl
   /// binding. The 'event' argument is the pointer event that was being routed.
   /// The 'target' argument is the class whose handleEvent function threw the
   /// exception. The 'exception' argument contains the object that was thrown,
-  /// and the 'stack' argument contains the stack trace. The callback is invoked
+  /// and the 'stack' argument contains the stack trace. If no handler is
-  /// after the information is printed to the console.
+  /// registered, then the information will be printed to the console instead.
   GesturerExceptionHandler debugGesturerExceptionHandler;
 
   /// Dispatch the given event to the path of the given hit test result
@@ -93,17 +93,20 @@ abstract class Gesturer extends BindingBase implements HitTestTarget, HitTestabl
       try {
         entry.target.handleEvent(event, entry);
       } catch (exception, stack) {
-        debugPrint('-- EXCEPTION --');
+        if (debugGesturerExceptionHandler != null) {
+          debugGesturerExceptionHandler(event, entry.target, exception, stack);
+        } else {
+          debugPrint('-- EXCEPTION CAUGHT BY GESTURE LIBRARY ---------------------------------');
           debugPrint('The following exception was raised while dispatching a pointer event:');
           debugPrint('$exception');
-        debugPrint('Stack trace:');
-        debugPrint('$stack');
           debugPrint('Event:');
           debugPrint('$event');
           debugPrint('Target:');
           debugPrint('${entry.target}');
-        if (debugGesturerExceptionHandler != null)
+          debugPrint('Stack trace:');
-          debugGesturerExceptionHandler(event, entry.target, exception, stack);
+          debugPrint('$stack');
+          debugPrint('------------------------------------------------------------------------');
+        }
       }
     }
   }

packages/flutter/lib/src/gestures/pointer_router.dart

@@ -43,9 +43,9 @@ class PointerRouter {
   /// the exception. The 'event' argument is the pointer event that was being
   /// routed. The 'route' argument is the callback that threw the exception. The
   /// 'exception' argument contains the object that was thrown, and the 'stack'
-  /// argument contains the stack trace. The callback is invoked after the
+  /// argument contains the stack trace. If no handler is registered, then the
-  /// information (exception, stack trace, and event; not the route callback
+  /// human-readable parts of this information (the exception, event, and stack
-  /// itself) is printed to the console.
+  /// trace) will be printed to the console instead.
   PointerExceptionHandler debugPointerExceptionHandler;
 
   /// Calls the routes registered for this pointer event.
@@ -60,15 +60,18 @@ class PointerRouter {
       try {
         route(event);
       } catch (exception, stack) {
-        debugPrint('-- EXCEPTION --');
+        if (debugPointerExceptionHandler != null) {
+          debugPointerExceptionHandler(this, event, route, exception, stack);
+        } else {
+          debugPrint('-- EXCEPTION CAUGHT BY GESTURE LIBRARY ---------------------------------');
           debugPrint('The following exception was raised while routing a pointer event:');
           debugPrint('$exception');
-        debugPrint('Stack trace:');
-        debugPrint('$stack');
           debugPrint('Event:');
           debugPrint('$event');
-        if (debugPointerExceptionHandler != null)
+          debugPrint('Stack trace:');
-          debugPointerExceptionHandler(this, event, route, exception, stack);
+          debugPrint('$stack');
+          debugPrint('------------------------------------------------------------------------');
+        }
       }
     }
   }

packages/flutter/lib/src/rendering/object.dart

@@ -381,9 +381,8 @@ typedef void RenderingExceptionHandler(RenderObject source, String method, dynam
 /// exception. The 'method' argument is the method in which the exception
 /// occurred; it will be one of 'performResize', 'performLayout, or 'paint'. The
 /// 'exception' argument contains the object that was thrown, and the 'stack'
-/// argument contains the stack trace. The callback is invoked after the
+/// argument contains the stack trace. If no handler is registered, then the
-/// information is printed to the console, and could be used to print additional
+/// information will be printed to the console instead.
-/// information, such as from [debugDumpRenderTree()].
 RenderingExceptionHandler debugRenderingExceptionHandler;
 
 /// An object in the render tree.
@@ -461,16 +460,19 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
 
   dynamic debugOwner;
   void _debugReportException(String method, dynamic exception, StackTrace stack) {
-    debugPrint('-- EXCEPTION --');
+    if (debugRenderingExceptionHandler != null) {
+      debugRenderingExceptionHandler(this, method, exception, stack);
+    } else {
+      debugPrint('-- EXCEPTION CAUGHT BY RENDERING LIBRARY -------------------------------');
       debugPrint('The following exception was raised during $method():');
       debugPrint('$exception');
-    debugPrint('Stack trace:');
-    debugPrint('$stack');
       debugPrint('The following RenderObject was being processed when the exception was fired:\n${this}');
       if (debugOwner != null)
         debugPrint('That RenderObject had the following owner:\n$debugOwner');
-    if (debugRenderingExceptionHandler != null)
+      debugPrint('Stack trace:');
-      debugRenderingExceptionHandler(this, method, exception, stack);
+      debugPrint('$stack');
+      debugPrint('------------------------------------------------------------------------');
+    }
   }
 
   static bool _debugDoingLayout = false;

packages/flutter/lib/src/scheduler/scheduler.dart

@@ -271,10 +271,12 @@ abstract class Scheduler extends BindingBase {
       if (debugSchedulerExceptionHandler != null) {
         debugSchedulerExceptionHandler(exception, stack);
       } else {
-        print('-- EXCEPTION IN SCHEDULER CALLBACK --');
+        debugPrint('-- EXCEPTION CAUGHT BY SCHEDULER LIBRARY -------------------------------');
-        print('$exception');
+        debugPrint('An exception was raised during a scheduler callback:');
-        print('Stack trace:');
+        debugPrint('$exception');
-        print('$stack');
+        debugPrint('Stack trace:');
+        debugPrint('$stack');
+        debugPrint('------------------------------------------------------------------------');
       }
     }
   }

packages/flutter/lib/src/services/image_resource.dart

@@ -5,6 +5,8 @@
 import 'dart:async';
 import 'dart:ui' as ui;
 
+import 'print.dart';
+
 /// A callback for when the image is available.
 typedef void ImageListener(ui.Image image);
 
@@ -15,7 +17,7 @@ typedef void ImageListener(ui.Image image);
 /// or because the underlying image resource was mutated.
 class ImageResource {
   ImageResource(this._futureImage) {
-    _futureImage.then(_handleImageLoaded, onError: _handleImageError);
+    _futureImage.then(_handleImageLoaded, onError: (exception, stack) => _handleImageError('Failed to load image:', exception, stack));
   }
 
   bool _resolved = false;
@@ -34,8 +36,13 @@ class ImageResource {
   /// this object will call the listener synchronously.
   void addListener(ImageListener listener) {
     _listeners.add(listener);
-    if (_resolved)
+    if (_resolved) {
+      try {
         listener(_image);
+      } catch (exception, stack) {
+        _handleImageError('The following exception was thrown by a synchronously-invoked image listener:', exception, stack);
+      }
+    }
   }
 
   /// Stop listening for new concrete [ui.Image] objects.
@@ -49,19 +56,24 @@ class ImageResource {
     _notifyListeners();
   }
 
-  void _handleImageError(e, stackTrace) {
-    print('Failed to load image: $e\nStack trace: $stackTrace');
-  }
-
   void _notifyListeners() {
     assert(_resolved);
     List<ImageListener> localListeners = new List<ImageListener>.from(_listeners);
     for (ImageListener listener in localListeners) {
       try {
         listener(_image);
-      } catch(e) {
+      } catch (exception, stack) {
-        print('Image listener had exception: $e');
+        _handleImageError('The following exception was thrown by an image listener:', exception, stack);
       }
     }
   }
+
+  void _handleImageError(String message, dynamic exception, dynamic stack) {
+    debugPrint('-- EXCEPTION CAUGHT BY SERVICES LIBRARY --------------------------------');
+    debugPrint(message);
+    debugPrint('$exception');
+    debugPrint('Stack trace:');
+    debugPrint('$stack');
+    debugPrint('------------------------------------------------------------------------');
+  }
 }

packages/flutter/lib/src/widgets/framework.dart

@@ -1404,6 +1404,7 @@ abstract class RenderObjectElement<T extends RenderObjectWidget> extends Buildab
   void mount(Element parent, dynamic newSlot) {
     super.mount(parent, newSlot);
     assert(_slot == newSlot);
+    assert(() { debugUpdateRenderObjectOwner(); return true; });
     attachRenderObject(newSlot);
     _dirty = false;
   }
@@ -1788,7 +1789,7 @@ void _debugReportException(String context, dynamic exception, StackTrace stack)
   if (debugWidgetsExceptionHandler != null) {
     debugWidgetsExceptionHandler(context, exception, stack);
   } else {
-    debugPrint('------------------------------------------------------------------------');
+    debugPrint('-- EXCEPTION CAUGHT BY WIDGETS LIBRARY ---------------------------------');
     debugPrint('Exception caught while $context');
     debugPrint('$exception');
     debugPrint('Stack trace:');