Improve exception reporting in Widgets framework

This specifically improves the reporting of exceptions in syncChild(),
and makes the way we've been adding information to toStringName() less
ad-hoc and easier to extend.

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

@@ -140,7 +140,6 @@ abstract class GlobalKey extends Key {
     if (_syncedKeys.isEmpty && _removedKeys.isEmpty)
       return;
     try {
-
       for (GlobalKey key in _syncedKeys) {
         Widget widget = _registry[key];
         if (widget != null && _syncListeners.containsKey(key)) {
@@ -149,7 +148,6 @@ abstract class GlobalKey extends Key {
             listener(key, widget);
         }
       }
-
       for (GlobalKey key in _removedKeys) {
         if (!_registry.containsKey(key) && _removeListeners.containsKey(key)) {
           Set<GlobalKeyRemoveListener> localListeners = new Set<GlobalKeyRemoveListener>.from(_removeListeners[key]);
@@ -388,6 +386,13 @@ abstract class Widget {
 
   // Returns the child which should be retained as the child of this node.
   Widget syncChild(Widget newNode, Widget oldNode, dynamic slot) {
+    String debugDetails;
+    assert(() {
+      // we save this information early because by the time the exception fires we might have changed everything around
+      debugDetails = "  old child: ${oldNode?.toStringName()}\n  new child: ${newNode?.toStringName()}";
+      return true;
+    });
+    try {
 
       if (newNode == oldNode) {
         // TODO(ianh): Simplify next few asserts once the analyzer is cleverer
@@ -483,6 +488,11 @@ abstract class Widget {
       newNode._sync(oldNode, slot);
       assert(newNode.renderObject is RenderObject);
       return newNode;
+
+    } catch (e, stack) {
+      _debugReportException('syncing children of ${this.toStringName()}\n$debugDetails', e, stack);
+      return null;
+    }
   }
 
   String _adjustPrefixWithParentCheck(Widget child, String prefix) {
@@ -507,19 +517,25 @@ abstract class Widget {
       nextPrefix = prefix + '   ';
       childrenString += lastChild.toString(nextPrefix, _adjustPrefixWithParentCheck(lastChild, nextStartPrefix));
     }
-    String suffix = '';
+    return '$startPrefix${toStringName()}\n$childrenString';
+  }
+  String toStringName() {
+    List<String> details = <String>[];
+    debugAddDetails(details);
+    String detailString = details.join('; ');
+    return '$runtimeType($detailString})';
+  }
+  void debugAddDetails(List<String> details) {
+    if (key != null)
+      details.add('$key');
+    details.add('hashCode=$hashCode');
+    details.add(mounted ? 'mounted' : 'not mounted');
+    String generationString = '';
     if (_generation != _currentGeneration) {
       int delta = _generation - _currentGeneration;
       String sign = delta < 0 ? '' : '+';
-      suffix = ' gen$sign$delta';
-    }
-    return '$startPrefix${toStringName()}$suffix\n$childrenString';
+      details.add('gen$sign$delta');
     }
-  String toStringName() {
-    String keyString = key == null ? '' : '$key; ';
-    String hashCodeString = 'hashCode=$hashCode';
-    String mountedString = mounted ? '; mounted' : '; not mounted';
-    return '$runtimeType($keyString$hashCodeString$mountedString)';
   }
 
   // This function can be safely called when the layout is valid.
@@ -902,6 +918,11 @@ abstract class Component extends Widget {
 
   Widget build();
 
+  void debugAddDetails(List<String> details) {
+    super.debugAddDetails(details);
+    if (_dirty)
+      details.add('dirty');
+  }
 }
 
 abstract class StatefulComponent extends Component {
@@ -964,10 +985,9 @@ abstract class StatefulComponent extends Component {
     _scheduleBuild();
   }
 
-  String toStringName() {
-    if (_isStateInitialized)
-      return 'Stateful ${super.toStringName()}';
-    return 'Stateless ${super.toStringName()}';
+  void debugAddDetails(List<String> details) {
+    super.debugAddDetails(details);
+    details.add(_isStateInitialized ? 'stateful' : 'stateless');
   }
 }