Compute frontend/backend specific constants.

tests/compiler/dart2js_extra/constant_javascript_semantics_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";
 
 // Make sure we use JavaScript semantics when compiling compile-time constants.
 
 const x = 12345678901234567891;
 const y = 12345678901234567890;
 const z = x - y;
 
 const a = 1.0;
-const b = a << 3;  // This would not be valid with Dart semantics.
+const b = a << 3;  /// 01: compile-time error
 
 const c = -0.0;
-const d = c << 1;  // This would not be valid with Dart semantics.
+const d = c << 1;  /// 02: compile-time error
 
 foo() => 12345678901234567891 - 12345678901234567890;
 
 main() {
   Expect.equals(0, z);
   Expect.equals(0, x - y);
   Expect.equals(0, foo());
   Expect.isTrue(x is double);
   Expect.isTrue(x is int);
-  Expect.equals(8, b);
-  Expect.equals(8, 1.0 << 3);  // This would not be valid with Dart semantics.
+  Expect.equals(8, b);  /// 01: continued
+  Expect.equals(8, 1.0 << 3);  /// 03: static type warning
   Expect.isTrue(1 == 1.0);
-  Expect.equals(0, d);
-  Expect.equals(0, -0.0 << 1);  // This would not be valid with Dart semantics.
+  Expect.equals(0, d);  /// 02: continued
+  Expect.equals(0, -0.0 << 1);  /// 04: static type warning
   // Make sure the 1 is not shifted into the 32 bit range.
   Expect.equals(0, 0x100000000 >> 3);
   // The dynamic int-check also allows -0.0.
   Expect.isTrue((-0.0) is int);
 }