Add optional message argument to assert statements in the VM.

tests/language/assert_message_test.dart

+// Copyright (c) 2016, 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.
+
+// SharedOptions=--assert-message
+
+import "dart:async";
+
+import "package:async_helper/async_helper.dart";
+import "package:expect/expect.dart";
+
+main() {
+  // Only run with asserts enabled mode.
+  bool assertsEnabled = false;
+  assert(assertsEnabled = true);
+  if (!assertsEnabled) return;
+
+  // Basics.
+  assert(true, "");
+  assert(() => true, "");
+
+  int x = null;
+  // Successful asserts won't execute message.

[regis, 2016/12/13 23:26:51]

Is this still true when await is involved?

[hausner, 2016/12/14 16:59:35]

Yes. Added test case below in the async test cases.

+  assert(true, x + 42);
+  assert(true, throw "unreachable");
+
+  // Can use any value as message.
+  try {
+    assert(false, 42);
+  } on AssertionError catch (e) {
+    Expect.equals(42, e.message);
+  }
+
+  try {
+    assert(false, "");
+  } on AssertionError catch (e) {
+    Expect.equals("", e.message);
+  }
+
+  try {
+    assert(false, null);
+  } on AssertionError catch (e) {
+    Expect.equals(null, e.message);
+  }
+
+  // Test expression can throw.
+  try {
+    assert(throw "test", throw "message");
+  } on String catch (e) {
+    Expect.equals("test", e);
+  }
+
+  // Message expression can throw.
+  try {
+    assert(false, throw "message");
+  } on String catch (e) {
+    Expect.equals("message", e);
+  }
+
+  // Failing asserts evaluate message after test.
+  var list = [];
+  try {
+    assert((list..add(1)).isEmpty, (list..add(3)).length);
+  } on AssertionError catch (e) {
+    Expect.equals(2, e.message);
+    Expect.listEquals([1, 3], list);
+  }
+
+  asyncStart();
+  asyncTests().then((_) { asyncEnd(); });
+}
+
+Future asyncTests() async {
+  // You can await in both condition and message.
+  assert(true, await 0);
+  assert(await true, 1);
+  assert(await true, await 2);

[regis, 2016/12/13 23:26:51]

How about
assert(await true, throw "unreachable");
and
assert(await true, await throw "unreachable");

[hausner, 2016/12/14 16:59:35]

Done.

+
+  try {
+    assert(false, await 3);
+  } on AssertionError catch (e) {
+    Expect.equals(3, e.message);
+  }
+
+  var falseFuture = new Future.value(false);
+  var numFuture = new Future.value(4);
+
+  try {
+    assert(await falseFuture, await numFuture);
+  } on AssertionError catch (e) {
+    Expect.equals(4, e.message);
+  }
+
+  try {
+    assert(await falseFuture, await new Future.error("error"));
+  } on String catch (e) {
+    Expect.equals("error", e);
+  }
+}