Fix #23871: identical on consts should be always const

tests/language/const_syntax_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.
 
 import "package:expect/expect.dart";
 
 main() {
   const f0 = 42;
   const f1; /// 01: compile-time error
   const int f2 = 87;
@@ skipping 14 matching lines @@
   Expect.isTrue(A0 is int);
   Expect.isTrue(A1 is int);
   Expect.isTrue(A2 is int); /// 08: compile-time error
   Expect.isTrue(A3 is int); /// 08: continued
 
   Expect.isTrue(C0.X is C1);
   Expect.isTrue(C0.X.x is C1); /// 09: compile-time error
 
   Expect.equals("Hello 42", B2);
   Expect.equals("42Hello", B3); /// 10: compile-time error
+
+  const cf1 = identical(const Point(1, 2), const Point(1, 2));
+
+  const cf2 = identical(const Point(1, 2), new Point(1, 2));  /// 11: compile-time error
+
+  var f4 = B4;   /// 12: compile-time error
+  var f5 = B5;
 }
 
 const F0 = 42;
 const F1;                 /// 03: continued
 const int F2 = 87;
 const int F3;             /// 04: continued
 
 class Point {
   final x, y;
   const Point(this.x, this.y);
@@ skipping 23 matching lines @@
       : x = C0.X /// 09: continued
   ;
   final x = null;
 }
 
 // Check that sub-expressions of binary + are numeric.
 const B0 = 42;
 const B1 = "Hello";
 const B2 = "$B1 $B0";
 const B3 = B0 + B1; /// 10: continued
+
+// Check identical.
+
+const B4 = identical(1, new Point(1,2));
+const B5 = identical(1, const Point(1,2));