Fix debugger's calculation of the context level in async code.

runtime/observatory/tests/service/regress_28443_test.dart

+// Copyright (c) 2017, 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.
+
+import 'dart:async';
+import 'test_helper.dart';
+import 'service_test_common.dart';
+import 'package:observatory/service_io.dart';
+import 'package:unittest/unittest.dart';
+
+const int LINE_A = 28, LINE_B = 33;
+
+class VMServiceClient {
+  VMServiceClient(this.x);
+  close() => new Future.microtask(() => print("close"));
+  var x;
+}
+
+collect() async {
+  var uri = "abc";
+  var vmService;
+  await new Future.microtask(() async {
+    try {
+      vmService = new VMServiceClient(uri);
+      await new Future.microtask(() => throw new TimeoutException("here"));
+    } on dynamic {
+      vmService.close();
+      rethrow;  // LINE_A
+    }
+  });
+}
+
+test_code() async {  // LINE_B
+  await collect();
+}
+
+Future<Isolate> stepThroughProgram(Isolate isolate) async {
+  Completer completer = new Completer();
+  int pauseEventsSeen = 0;
+
+  await subscribeToStream(isolate.vm, VM.kDebugStream,
+      (ServiceEvent event) async {
+    if (event.kind == ServiceEvent.kPauseBreakpoint) {
+      // We are paused: Step further.
+      pauseEventsSeen++;
+      isolate.stepInto();
+    } else if (event.kind == ServiceEvent.kPauseExit) {
+      // We are at the exit: The test is done.
+      expect(pauseEventsSeen > 20, true,
+          reason: "Saw only $pauseEventsSeen pause events.");
+      await cancelStreamSubscription(VM.kDebugStream);
+      completer.complete();
+    }
+  });
+  isolate.resume();
+  return completer.future;
+}
+
+var tests = [hasPausedAtStart,
+             setBreakpointAtLine(LINE_B),
+             resumeIsolate,
+             hasStoppedAtBreakpoint,
+             setBreakpointAtLine(LINE_A),
+             resumeIsolate,
+             hasStoppedAtBreakpoint,
+             stepOut,
+             stoppedAtLine(LINE_B),
+             resumeIsolate];
+
+main(args) => runIsolateTestsSynchronous(args, tests,
+    testeeConcurrent: test_code, pause_on_start: true, pause_on_exit: false);