Add assert-in-initializer-list to formal specification.

tests/language/assertion_initializer_const_error_test.dart

+// Copyright (c) 201, 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=--assert_initializer
+
+class C {
+  static bool check(x, y) => x < y;
+  final int x;
+  const C(this.x);
+  // The expression is not a potentially constant expression.
+  // This is a compile-time error.
+  const C.bc03(this.x, y) : assert(check(x, y));  //# 01: compile-time error

[floitsch, 2017/06/29 12:08:56]

ditto: move the "assert(check(x, y))" on a separate line. Then leave the call
below unaffected.

+}
+
+main() {
+  var c = new C(1);
+  // Ensure that the constructor is compiled.
+  c = new C.bc03(1, 2);  //# 01: continued
+  if (c.x != 1) throw "non-trivial use of c";
+}