Fix hot reload (#5799)

I forgot that it was possible for the root view to get marked dirty
without getting a new widget. This fixes that case to work.

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

@@ -8,6 +8,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/scheduler.dart';
 import 'package:flutter/services.dart';
+import 'package:meta/meta.dart';
 import 'package:mojo/core.dart' as core;
 import 'package:sky_services/semantics/semantics.mojom.dart' as mojom;
 
@@ -195,6 +196,7 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   /// list.
   //
   // When editing the above, also update widgets/binding.dart's copy.
+  @protected
   void beginFrame() {
     assert(renderView != null);
     pipelineOwner.flushLayout();
@@ -208,7 +210,7 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   void reassembleApplication() {
     super.reassembleApplication();
     pipelineOwner.reassemble();
-    beginFrame();
+    handleBeginFrame(null);
   }
 
   @override

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

@@ -491,11 +491,14 @@ class RenderObjectToWidgetElement<T extends RenderObject> extends RootRenderObje
 
   @override
   void performRebuild() {
-    assert(_newWidget != null);
-    final Widget newWidget = _newWidget;
-    _newWidget = null;
+    if (_newWidget != null) {
+      // _newWidget can be null if, for instance, we were rebuilt
+      // due to a reassemble.
+      final Widget newWidget = _newWidget;
+      _newWidget = null;
+      update(newWidget);
+    }
     super.performRebuild();
-    update(newWidget);
     assert(_newWidget == null);
   }
 

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

@@ -1537,15 +1537,25 @@ class BuildOwner {
         assert(_dirtyElements[index] != null);
         assert(_dirtyElements[index]._inDirtyList);
         assert(!_dirtyElements[index]._active || _dirtyElements[index]._debugIsInScope(context));
-        _dirtyElements[index].rebuild();
+        try {
+          _dirtyElements[index].rebuild();
+        } catch (e, stack) {
+          _debugReportException(
+            'while rebuilding dirty elements', e, stack,
+            informationCollector: (StringBuffer information) {
+              information.writeln('The element being rebuilt at the time was index $index of $dirtyCount:');
+              information.write('  ${_dirtyElements[index]}');
+            }
+          );
+        }
         index += 1;
         if (dirtyCount < _dirtyElements.length) {
           _dirtyElements.sort(_elementSort);
           dirtyCount = _dirtyElements.length;
         }
       }
-    } finally {
       assert(!_dirtyElements.any((BuildableElement element) => element._active && element.dirty));
+    } finally {
       for (BuildableElement element in _dirtyElements) {
         assert(element._inDirtyList);
         element._inDirtyList = false;
@@ -3149,11 +3159,14 @@ class MultiChildRenderObjectElement extends RenderObjectElement {
   }
 }
 
-void _debugReportException(String context, dynamic exception, StackTrace stack) {
+void _debugReportException(String context, dynamic exception, StackTrace stack, {
+  InformationCollector informationCollector
+}) {
   FlutterError.reportError(new FlutterErrorDetails(
     exception: exception,
     stack: stack,
     library: 'widgets library',
-    context: context
+    context: context,
+    informationCollector: informationCollector,
   ));
 }

packages/flutter/test/widget/reassemble_test.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('reassemble does not crash', (WidgetTester tester) async {
+    await tester.pumpWidget(new MaterialApp(
+      home: new Text('Hello World')
+    ));
+    await tester.pump();
+    tester.binding.reassembleApplication();
+    await tester.pump();
+  });
+}