Add tests of line numbers to causal async stack tests

runtime/observatory/tests/service/causal_async_stack_contents_test.dart

Index: runtime/observatory/tests/service/causal_async_stack_contents_test.dart
diff --git a/runtime/observatory/tests/service/causal_async_stack_contents_test.dart b/runtime/observatory/tests/service/causal_async_stack_contents_test.dart
index 9777993ca9d9bbf0a3271733a2e9e11de55d9391..cfdc667aa9d354a17a576bfc6b4479c3cef040af 100644
--- a/runtime/observatory/tests/service/causal_async_stack_contents_test.dart
+++ b/runtime/observatory/tests/service/causal_async_stack_contents_test.dart
@@ -65,6 +65,12 @@ var tests = [
     expect(asyncStack[1].toString(), contains('helper'));
     expect(asyncStack[2].kind, equals(M.FrameKind.asyncSuspensionMarker));
     expect(asyncStack[3].toString(), contains('testMain'));
+    // Line 19.
+    expect(await asyncStack[0].location.toUserString(), contains('.dart:19'));
+    // Line 25.
+    expect(await asyncStack[1].location.toUserString(), contains('.dart:25'));
+    // Line 30.
+    expect(await asyncStack[3].location.toUserString(), contains('.dart:30'));
   },
 ];