Improvements to causal async stack traces

tests/language/vm/causal_async_exception_stack2_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 'package:unittest/unittest.dart';
+
+foo3() async => throw "foo";
+bar3() async => throw "bar";
+
+foo2() async => foo3();
+bar2() async => bar3();
+
+foo() async => foo2();
+bar() async => bar2();
+
+test1() async {
+  // test1 -> foo -> foo2 -> foo3
+  // test1 -> bar -> bar2 -> bar3
+  // These run interleaved, check their stack traces don't become mixed.
+  var a = foo();
+  var b = bar();
+
+  try {
+    await a;
+  } catch(e, st) {
+    // st has foo,2,3 and not bar,2,3.
+    expect(st.toString(), stringContainsInOrder([
+           'foo3',
+           '<asynchronous suspension>',
+           'foo2',
+           '<asynchronous suspension>',
+           'foo',
+           '<asynchronous suspension>',
+           'test1',
+           ]));
+    expect(st.toString().contains('bar'), isFalse);
+  }
+
+  try {
+    await b;
+  } catch(e, st) {
+    // st has bar,2,3 but not foo,2,3
+    expect(st.toString(), stringContainsInOrder([
+           'bar3',
+           '<asynchronous suspension>',
+           'bar2',
+           '<asynchronous suspension>',
+           'bar',
+           '<asynchronous suspension>',
+           'test1',
+           ]));
+    expect(st.toString().contains('foo'), isFalse);
+  }
+}
+
+test2() async {
+  // test2 -> foo -> foo2 -> foo3
+  // test2 -> bar -> bar2 -> bar3
+  // These run sequentially, check the former stack trace didn't get linked to
+  // from the latter stack trace.
+
+  try {
+    await foo();
+  } catch(e, st) {
+    // st has foo,2,3 but not bar,2,3
+    expect(st.toString(), stringContainsInOrder([
+           'foo3',
+           '<asynchronous suspension>',
+           'foo2',
+           '<asynchronous suspension>',
+           'foo',
+           '<asynchronous suspension>',
+           'test2',
+           ]));
+    expect(st.toString().contains('bar'), isFalse);
+  }
+
+  try {
+    await bar();
+  } catch(e, st) {
+    // st has bar,2,3 but not foo,2,3
+    expect(st.toString(), stringContainsInOrder([
+           'bar3',
+           '<asynchronous suspension>',
+           'bar2',
+           '<asynchronous suspension>',
+           'bar',
+           '<asynchronous suspension>',
+           'test2',
+           ]));
+    expect(st.toString().contains('foo'), isFalse);
+  }
+}
+
+main() async {
+  await test1();
+  await test2();
+}