Track the 'awaiter return' call stack use it to detect uncaught exceptions in async functions

runtime/observatory/tests/service/pause_on_unhandled_async_exceptions2_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.
+// VMOptions=--error_on_bad_type --error_on_bad_override
+
+import 'package:observatory/service_io.dart';
+import 'package:observatory/models.dart' as M;
+import 'package:unittest/unittest.dart';
+import 'test_helper.dart';
+import 'service_test_common.dart';
+
+const LINE_A = 29;
+
+class Foo {
+
+}
+
+doThrow() {
+  throw "TheException"; // Line 13.
+  return "end of doThrow";
+}
+
+asyncThrower() async {
+  doThrow();
+}
+
+testeeMain() async {
+  // No try ... catch.
+  await asyncThrower();
+}
+
+var tests = [
+  hasStoppedWithUnhandledException,
+
+  (Isolate isolate) async {
+    print("We stoppped!");
+    var stack = await isolate.getStack();
+    expect(stack['asyncCausalFrames'], isNotNull);
+    var asyncStack = stack['asyncCausalFrames'];
+    expect(asyncStack[0].toString(), contains('doThrow'));
+    expect(asyncStack[1].toString(), contains('asyncThrower'));
+    expect(asyncStack[2].kind, equals(M.FrameKind.asyncSuspensionMarker));
+    expect(asyncStack[3].toString(), contains('testeeMain'));
+    // We've stopped at LINE_A.
+    expect(await asyncStack[3].location.toUserString(),
+           contains('.dart:$LINE_A'));
+  }
+];
+
+main(args) => runIsolateTests(args,
+                              tests,
+                              pause_on_unhandled_exceptions: true,
+                              testeeConcurrent: testeeMain);