Filter frames from debugger stack based on visibility rather than "debuggability" (steppability).

tests/standalone/vmservice/isolate_stacktrace_command_test.dart

Index: tests/standalone/vmservice/isolate_stacktrace_command_test.dart
diff --git a/tests/standalone/vmservice/isolate_stacktrace_command_test.dart b/tests/standalone/vmservice/isolate_stacktrace_command_test.dart
index 4b4ce79f56d999ad3457eb88140ef936d8f4a71c..03caed145c5de1480a6655270490421f1667a31b 100644
--- a/tests/standalone/vmservice/isolate_stacktrace_command_test.dart
+++ b/tests/standalone/vmservice/isolate_stacktrace_command_test.dart
@@ -13,7 +13,10 @@ class StacktraceTest extends VmServiceRequestHelper {
 
   onRequestCompleted(Map reply) {
     Expect.equals('StackTrace', reply['type'], 'Not a StackTrace message.');
-    Expect.equals(4, reply['members'].length, 'Stacktrace is wrong length.');
+    Expect.isTrue(4 <= reply['members'].length, 'Stacktrace is wrong length.');
+    // The number of frames involved in isolate message dispatch is an
+    // implementation detail. Only check that we got all the frames for user
+    // code.
     Expect.equals('a', reply['members'][0]['name']);
     Expect.equals('b', reply['members'][1]['name']);
     Expect.equals('c', reply['members'][2]['name']);