Implement checked mode for const constructors.

tests/language/compile_time_constant_checked2_test.dart

+// Copyright (c) 2012, 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.
+
+class A {
+  final int x;
+  const A.a1() : x = 'foo';
+  const A.a2(this.x);
+  const A.a3([this.x = 'foo']);
+  const A.a4(String this.x);
+  const A.a5(String x) : this.x = x;
+  const A.a6(int x) : this.x = x;
+}
+
+const a1 = const A.a1(); /// 01: compile-time error
+const a2 = const A.a2('foo'); /// 02: compile-time error
+const a3 = const A.a3(); /// 03: compile-time error
+const a4 = const A.a4('foo'); /// 04: compile-time error
+const a5 = const A.a5('foo'); /// 05: compile-time error
+const a6 = const A.a6('foo'); /// 06: compile-time error
+
+var b1 = const A.a1(); /// 07: compile-time error
+var b2 = const A.a2('foo'); /// 08: compile-time error
+var b3 = const A.a3(); /// 09: compile-time error
+var b4 = const A.a4('foo'); /// 10: compile-time error
+var b5 = const A.a5('foo'); /// 11: compile-time error
+var b6 = const A.a6('foo'); /// 12: compile-time error
+
+main() {
+  print(a1); /// 01: continued
+  print(a2); /// 02: continued
+  print(a3); /// 03: continued
+  print(a4); /// 04: continued
+  print(a5); /// 05: continued
+  print(a6); /// 06: continued
+
+  print(b1); /// 07: continued
+  print(b2); /// 08: continued
+  print(b3); /// 09: continued
+  print(b4); /// 10: continued
+  print(b5); /// 11: continued
+  print(b6); /// 12: continued
+}