[VM/Debugger] Make Debugger::FindBestFit skip functions with non-real start or end positions

TEST=confirmed that pkg/vm_service/test/regress_60396_test passes with
the changes in this CL and fails without them

Fixes: #60396
Change-Id: I093fa3abf98b41c47786f0d37987ae771efe072e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420880
Commit-Queue: Derek Xu <[email protected]>
Reviewed-by: Ben Konyi <[email protected]>

Author: derekxu16

pkg/pkg.status

@@ -190,6 +190,7 @@ vm_service/test/regress_46419_test: SkipByDesign # Debugger is disabled in AOT m
 vm_service/test/regress_46559_test: SkipByDesign # Debugger is disabled in AOT mode.
 vm_service/test/regress_48279_test: SkipByDesign # Debugger is disabled in AOT mode.
 vm_service/test/regress_55559_test: SkipByDesign # Spawns a child process from source.
+vm_service/test/regress_60396_test: SkipByDesign # Debugger is disabled in AOT mode.
 vm_service/test/regress_88104_test: SkipByDesign # Debugger is disabled in AOT mode.
 vm_service/test/reload_sources_rpc_triggers_isolate_reload_event_test: SkipByDesign # Hot reload is disabled in AOT mode.
 vm_service/test/reload_sources_test: SkipByDesign # Hot reload is disabled in AOT mode.

pkg/vm_service/test/regress_60396_test.dart

@@ -0,0 +1,49 @@
+// Copyright (c) 2025, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// This is a regression test for https://dartbug.com/60396.
+
+import 'common/service_test_common.dart';
+import 'common/test_helper.dart';
+
+// AUTOGENERATED START
+//
+// Update these constants by running:
+//
+// dart pkg/vm_service/test/update_line_numbers.dart pkg/vm_service/test/regress_60396_test.dart
+//
+const LINE_A = 22;
+// AUTOGENERATED END
+
+void testeeMain() {
+  (() {
+    (() {
+      return 1 + 2; // LINE_A
+    })();
+  })();
+}
+
+// We define this enum because the bug that this test is a regression test
+// against was that Debugger::FindBestFit would sometimes attempt to resolve
+// breakpoints in functions with non-real token positions like [_enumToString].
+enum MyEnum { A, B }
+
+final tests = <IsolateTest>[
+  hasPausedAtStart,
+  setBreakpointAtLine(LINE_A),
+  resumeIsolate,
+  hasStoppedAtBreakpoint,
+  stoppedAtLine(LINE_A),
+  resumeIsolate,
+  hasStoppedAtExit,
+];
+
+void main([args = const <String>[]]) => runIsolateTests(
+      args,
+      tests,
+      'regress_60396_test.dart',
+      testeeConcurrent: testeeMain,
+      pauseOnStart: true,
+      pauseOnExit: true,
+    );

runtime/vm/debugger.cc

@@ -2505,7 +2505,8 @@ bool Debugger::FindBestFit(const Script& script,
           function ^= functions.At(pos);
           ASSERT(!function.IsNull());
           if (IsImplicitFunction(function) || function.is_synthetic() ||
-              !function.is_debuggable()) {
+              !function.token_pos().IsReal() ||
+              !function.end_token_pos().IsReal() || !function.is_debuggable()) {
             // We skip implicit functions and synthetic functions because they
             // do not have user specifiable source locations. We also skip
             // functions marked as undebuggable.