Migrator: Begin flow analysis on top-level annotations

Fixes #42295

Change-Id: I9d224b5c8a3fa5d57f17e434a9e6dfbcebc14781
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151026
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Paul Berry <[email protected]>

Author: srawlins

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -127,7 +127,7 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
       _flowAnalysis;
 
   /// If we are visiting a function body or initializer, assigned variable
-  /// information  used in flow analysis.  Otherwise `null`.
+  /// information used in flow analysis.  Otherwise `null`.
   AssignedVariables<AstNode, PromotableElement> _assignedVariables;
 
   /// The [DecoratedType] of the innermost function or method being visited, or
@@ -281,6 +281,32 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
     }
   }
 
+  @override
+  // TODO(srawlins): Theoretically, edges should be connected between arguments
+  // and parameters, as in an instance creation. It is quite rare though, to
+  // declare a class and use it as an annotation in the same package.
+  DecoratedType visitAnnotation(Annotation node) {
+    var previousFlowAnalysis = _flowAnalysis;
+    var previousAssignedVariables = _assignedVariables;
+    if (_flowAnalysis == null) {
+      _assignedVariables = AssignedVariables();
+      _flowAnalysis = FlowAnalysis<AstNode, Statement, Expression,
+              PromotableElement, DecoratedType>(
+          DecoratedTypeOperations(_typeSystem, _variables, _graph),
+          _assignedVariables);
+    }
+    try {
+      _dispatch(node.name);
+      _dispatch(node.constructorName);
+      _dispatchList(node.arguments?.arguments);
+    } finally {
+      _flowAnalysis = previousFlowAnalysis;
+      _assignedVariables = previousAssignedVariables;
+    }
+    annotationVisited(node);
+    return null;
+  }
+
   @override
   DecoratedType visitAsExpression(AsExpression node) {
     if (BestPracticesVerifier.isUnnecessaryCast(

pkg/nnbd_migration/lib/src/utilities/completeness_tracker.dart

@@ -19,11 +19,15 @@ mixin CompletenessTracker<T> on AstVisitor<T>, PermissiveModeVisitor<T> {
 
   @override
   T visitAnnotation(Annotation node) {
+    annotationVisited(node);
+    return super.visitAnnotation(node);
+  }
+
+  void annotationVisited(Annotation node) {
     assert(() {
       _annotationTracker.nodeVisited(node);
       return true;
     }());
-    return super.visitAnnotation(node);
   }
 
   void typeNameVisited(TypeName node) {

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -7508,6 +7508,16 @@ int f() {
     expect(edge.sourceNode.displayName, 'throw expression (test.dart:2:10)');
   }
 
+  Future<void> test_top_level_annotation_begins_flow_analysis() async {
+    await analyze('''
+class C {
+  const C(bool x);
+}
+@C(true)
+int x;
+''');
+  }
+
   Future<void> test_topLevelSetter() async {
     await analyze('''
 void set x(int value) {}