Report a compile-time error if a const field is multiply initialized.

tests/language/const_error_multiply_initialized_test.dart

+// Copyright (c) 2015, 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.
+
+// If a constant constructor contains an initializer, or an initializing
+// formal, for a final field which itself has an initializer at its
+// declaration, then a runtime error should occur if that constructor is
+// invoked using "new", but there should be no compile-time error.  However, if
+// the constructor is invoked using "const", there should be a compile-time
+// error, since it is a compile-time error for evaluation of a constant object
+// to result in an uncaught exception.
+
+import "package:expect/expect.dart";
+
+class C {
+  final x = 1;
+  const C() : x = 2; /// 01: compile-time error
+  const C() : x = 2; /// 02: static type warning
+  const C(this.x); /// 03: compile-time error
+  const C(this.x); /// 04: static type warning
+}
+
+main() {
+  const C(); /// 01: continued
+  Expect.throws(() => new C()); /// 02: continued
+  const C(2); /// 03: continued
+  Expect.throws(() => new C(2)); /// 04: continued
+}