Implement messages in assert() in dev_compiler.

tests/language_strong/assert_with_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.
+
+import "package:expect/expect.dart";
+
+main() {
+  var assertsEnabled = false;
+  assert((assertsEnabled = true));
+  if (!assertsEnabled) return;
+
+  // TODO(rnystrom): Test cases where the first argument to assert() is a
+  // function.
+
+  testAssertFails();
+  testAssertDoesNotFail();
+  testDoesNotEvaluateMessageIfAssertSucceeds();
+  testCallsToStringOnMessageLazily();
+}
+
+/// A class with a custom toString() that tracks when it is called.
+class ToString {
+  bool calledToString = false;
+
+  String toString() {
+    calledToString = true;
+    return "toString!";
+  }
+}
+
+testAssertFails() {
+  try {
+    assert(false, "Oops");
+    Expect.fail("Assert should throw.");
+  } catch (e) {
+    Expect.isTrue(e.toString().contains("Oops"));
+  }
+}
+
+testAssertDoesNotFail() {
+  try {
+    assert(true, "Oops");
+  } catch (e) {
+    Expect.fail("Assert should not throw.");
+  }
+}
+
+testDoesNotEvaluateMessageIfAssertSucceeds() {
+  try {
+    var evaluated = false;
+    assert(true, evaluated = true);
+    Expect.isFalse(evaluated);
+  } catch (e) {
+    Expect.fail("Assert should not throw.");
+  }
+}
+
+testCallsToStringOnMessageLazily() {
+  var toString = new ToString();
+  try {
+    assert(false, toString);
+    Expect.fail("Assert should throw.");
+  } catch (e) {
+    Expect.isFalse(toString.calledToString);
+    Expect.isTrue(e.toString().contains("toString!"));
+    Expect.isTrue(toString.calledToString);
+  }
+}