Migrator: Do not connect return values to return types on generators

Fixes #42299

Change-Id: I07b3955907efa3007ec146f4cb4170b117b58dcf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151328
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

Author: srawlins

pkg/nnbd_migration/lib/src/edge_builder.dart

@@ -1362,7 +1362,12 @@ class EdgeBuilder extends GeneralizingAstVisitor<DecoratedType>
   DecoratedType visitReturnStatement(ReturnStatement node) {
     DecoratedType returnType = _currentFunctionType.returnType;
     Expression returnValue = node.expression;
-    final isAsync = node.thisOrAncestorOfType<FunctionBody>().isAsynchronous;
+    var functionBody = node.thisOrAncestorOfType<FunctionBody>();
+    if (functionBody.isGenerator) {
+      // Do not connect the return value to the return type.
+      return _dispatch(returnValue);
+    }
+    final isAsync = functionBody.isAsynchronous;
     if (returnValue == null) {
       var target =
           NullabilityNodeTarget.text('implicit null return').withCodeRef(node);

pkg/nnbd_migration/test/edge_builder_test.dart

@@ -6836,6 +6836,26 @@ int f() {
     expect(edge.sourceNode.displayName, 'implicit null return (test.dart:2:3)');
   }
 
+  Future<void> test_return_in_asyncStar() async {
+    await analyze('''
+Stream<int> f() async* {
+  yield 1;
+  return;
+}
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('Stream<int>').node);
+  }
+
+  Future<void> test_return_in_syncStar() async {
+    await analyze('''
+Iterable<int> f() sync* {
+  yield 1;
+  return;
+}
+''');
+    assertNoUpstreamNullability(decoratedTypeAnnotation('Iterable<int>').node);
+  }
+
   Future<void> test_return_null() async {
     await analyze('''
 int f() {