Make dominator nullable.

TEST="tested with the added test 'root-dominator-is-null'"

Bug: https://github.com/flutter/devtools/issues/3025
Change-Id: Ifd6f926e3ecd486d383b03e8b81289310a653bb4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/230882
Reviewed-by: Slava Egorov <vegorov@google.com>
Reviewed-by: Kenzie (Schmoll) Davisson <kenzieschmoll@google.com>
Commit-Queue: Kenzie (Schmoll) Davisson <kenzieschmoll@google.com>

pkg/vm_snapshot_analysis/CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+## 0.7.1
+
+- Make `CallGraphNode.dominator` nullable.
+
 ## 0.7.0
 
 - Migrate to null-safety.

pkg/vm_snapshot_analysis/lib/precompiler_trace.dart

@@ -39,7 +39,7 @@ class CallGraphNode {
   /// Dominator of this node.
   ///
   /// Computed by [CallGraph.computeDominators].
-  late CallGraphNode dominator;
+  CallGraphNode? dominator;
 
   /// Nodes dominated by this node.
   ///

pkg/vm_snapshot_analysis/pubspec.yaml

@@ -1,6 +1,6 @@
 name: vm_snapshot_analysis
 description: Utilities for analysing AOT snapshot size.
-version: 0.7.0
+version: 0.7.1
 
 homepage: https://github.com/dart-lang/sdk/tree/master/pkg/vm_snapshot_analysis
 

pkg/vm_snapshot_analysis/test/precompiler_trace_test.dart

@@ -103,6 +103,7 @@ void main() async {
     test('collapse-by-package', () async {
       await withFlag(testSource, '--trace_precompiler_to', (json) async {
         final jsonRaw = await loadJson(File(json));
+
         final callGraph = loadTrace(jsonRaw).collapse(NodeType.packageNode);
 
         // Collapsing by package should not collapse dart:* libraries into root
@@ -110,5 +111,15 @@ void main() async {
         expect(callGraph.root.pred, isEmpty);
       });
     });
+
+    test('root-dominator-is-null', () async {
+      await withFlag(testSource, '--trace_precompiler_to', (json) async {
+        final jsonRaw = await loadJson(File(json));
+
+        final callGraph = loadTrace(jsonRaw).collapse(NodeType.classNode);
+
+        expect(callGraph.root.dominator, isNull);
+      });
+    });
   });
 }