Export snapshot. (flutter#7197)

Primary purpose of this PR is to implement export for snapshots.

For release notes:
<img width="233" alt="Screenshot 2024-03-07 at 11 01 08�AM" src="https://github.com/flutter/devtools/assets/12115586/6889070b-febf-48f3-8456-cf9081b0d1bc">

Side effects of this PR are:
1. Refactored structure of snapshot diffing, because we want to store the snapshot in original format
2. Improved performance of snapshotting (increased number of snapshots that can be taken 10 times and performance 2 times).

Also this PR suggests to un-ignore DEPENDENCIES.md for folders where they help to understand code. 

Performance for DevTools in release mode, in chrome, connected to and on mac in debug mode, home page, on polina-c's mac:

1. Gallery, taking snapshots sequentially, one after another:
- Original: 3 snapshots takes 30 sec, crash happens on 6th snapshot
- New, before upgrade of vm_service: three snapshots 15 sec, OOM crash on 60th snapshot  

2. DevTools, with every screen opened, memory chart collapsed (per activity monitor takes almost 3 GB):
- Original: crash on first snapshot
- New: first snapshot takes ~10 min, crash on second one

TODO for this PR:

packages/.vscode/launch.json

@@ -49,6 +49,13 @@
                 "--dart-define=enable_experiments=true"
             ],
         },
+        {
+            "name": "devtools + release",
+            "request": "launch",
+            "type": "dart",
+            "program": "devtools_app/lib/main.dart",
+            "flutterMode": "release",
+        },
         {
             "name": "memory/default",
             "request": "launch",

packages/devtools_app/lib/src/screens/memory/framework/connected/memory_controller.dart

@@ -11,24 +11,23 @@ import 'package:vm_service/vm_service.dart';
 
 import '../../../../shared/globals.dart';
 import '../../../../shared/memory/class_name.dart';
+import '../../../../shared/memory/heap_graph_loader.dart';
 import '../../../../shared/utils.dart';
 import '../../panes/chart/primitives.dart';
 import '../../panes/diff/controller/diff_pane_controller.dart';
 import '../../panes/profile/profile_pane_controller.dart';
 import '../../panes/tracing/tracing_pane_controller.dart';
-import '../../shared/heap/model.dart';
 import '../../shared/primitives/memory_timeline.dart';
 import 'memory_protocol.dart';
 
 class MemoryFeatureControllers {
-  /// [diffPaneController] is passed for testability.
+  /// Controllers are passed for testability.
   MemoryFeatureControllers(
     DiffPaneController? diffPaneController,
     ProfilePaneController? profilePaneController,
   ) {
     memoryTimeline = MemoryTimeline();
-    diff = diffPaneController ??
-        DiffPaneController(SnapshotTakerRuntime(memoryTimeline));
+    diff = diffPaneController ?? _createDiffController();
     profile = profilePaneController ?? ProfilePaneController();
   }
 
@@ -37,9 +36,12 @@ class MemoryFeatureControllers {
   late MemoryTimeline memoryTimeline;
   TracingPaneController tracing = TracingPaneController();
 
+  DiffPaneController _createDiffController() =>
+      DiffPaneController(HeapGraphLoaderRuntime(memoryTimeline));
+
   void reset() {
     diff.dispose();
-    diff = DiffPaneController(SnapshotTakerRuntime(memoryTimeline));
+    diff = _createDiffController();
 
     profile.dispose();
     profile = ProfilePaneController();

packages/devtools_app/lib/src/screens/memory/panes/diff/.gitignore

@@ -0,0 +1 @@
+!DEPENDENCIES.md

packages/devtools_app/lib/src/screens/memory/panes/diff/DEPENDENCIES.md

@@ -0,0 +1,19 @@
+<!---
+Generated by https://github.com/polina-c/layerlens
+Dependencies that create loops (inversions) are marked with `!`.
+-->
+
+```mermaid
+flowchart TD;
+controller-->data;
+diff_pane.dart-->controller;
+diff_pane.dart-->widgets;
+widgets-->controller;
+widgets-->data;
+```
+
+### Inversions
+In this folder: 0
+
+Including sub-folders: 0
+

packages/devtools_app/lib/src/screens/memory/panes/diff/controller/DEPENDENCIES.md

@@ -0,0 +1,16 @@
+<!---
+Generated by https://github.com/polina-c/layerlens
+Dependencies that create loops (inversions) are marked with `!`.
+-->
+
+```mermaid
+flowchart TD;
+diff_pane_controller.dart-->class_data.dart;
+diff_pane_controller.dart-->item_controller.dart;
+```
+
+### Inversions
+In this folder: 0
+
+Including sub-folders: 0
+

packages/devtools_app/lib/src/screens/memory/panes/diff/controller/class_data.dart

@@ -4,9 +4,10 @@
 
 import 'package:flutter/foundation.dart';
 
-import '../../../../../shared/memory/adapted_heap_data.dart';
+import '../../../../../shared/memory/classes.dart';
+import '../../../../../shared/memory/heap_object.dart';
+import '../../../../../shared/memory/retaining_path.dart';
 import '../../../shared/heap/class_filter.dart';
-import '../../../shared/heap/heap.dart';
 import '../../../shared/primitives/simple_elements.dart';
 import '../data/classes_diff.dart';
 
@@ -35,13 +36,13 @@ class ClassesTableSingleData {
   final ClassFilterData filterData;
 
   /// Selected class.
-  final selection = ValueNotifier<SingleClassStats?>(null);
+  final selection = ValueNotifier<SingleClassData?>(null);
 }
 
 class ClassesTableDiffData {
   ClassesTableDiffData({
-    required this.before,
-    required this.after,
+    required this.heapBefore,
+    required this.heapAfter,
     required this.filterData,
   });
 
@@ -52,14 +53,40 @@ class ClassesTableDiffData {
   // to subscribe for the changes, for performance reasons.
 
   /// Function to get selected first heap to diff.
-  final HeapDataCallback before;
+  final HeapDataCallback heapBefore;
 
   /// Function to get selected second heap to diff.
-  final HeapDataCallback after;
+  final HeapDataCallback heapAfter;
 
   /// Current class filter data.
   final ClassFilterData filterData;
 
   /// Selected class.
   final selection = ValueNotifier<DiffClassData?>(null);
 }
+
+/// Data for visualization of a path.
+///
+/// Is instantiated only to visualize the selected path,
+/// not for each path in snapshot.
+class PathData {
+  PathData(this.classData, this.path);
+
+  final ClassData classData;
+  final PathFromRoot path;
+
+  ObjectSetStats get objects => classData.byPath[path]!;
+
+  @override
+  bool operator ==(Object other) {
+    if (other.runtimeType != runtimeType) {
+      return false;
+    }
+    return other is PathData &&
+        other.classData.className == classData.className &&
+        other.path == path;
+  }
+
+  @override
+  int get hashCode => Object.hash(classData.className, path);
+}