Ensure that a javascript compatibility warning results in a

tests/standalone/javascript_compatibility_errors_test.dart

 // Copyright (c) 2014, 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=--warn_on_javascript_compatibility --warning_as_error --optimization_counter_threshold=5
 
 import "package:expect/expect.dart";
 
 f(x, y) {
   // Unoptimized and optimized code.
-  1 is double;  /// 00: compile-time error
-  if (1 is double) { x++; }  /// 01: compile-time error
-  try { 1 as double; } on CastError catch (e) { }  /// 02: compile-time error
-  try { var y = 1 as double; } on CastError catch (e) { }  /// 03: compile-time error
-  1.0 is int;  /// 04: compile-time error
-  if (1.0 is int) { x++; }  /// 05: compile-time error
-  try { 1.0 as int; } on CastError catch (e) { }  /// 06: compile-time error
-  try { var z = 1.0 as int; } on CastError catch (e) { }  /// 07: compile-time error
+  1 is double;  /// 00: ok
+  if (1 is double) { x++; }  /// 01: ok
+  try { 1 as double; } on CastError catch (e) { }  /// 02: ok
+  try { var y = 1 as double; } on CastError catch (e) { }  /// 03: ok
+  1.0 is int;  /// 04: ok
+  if (1.0 is int) { x++; }  /// 05: ok
+  try { 1.0 as int; } on CastError catch (e) { }  /// 06: ok
+  try { var z = 1.0 as int; } on CastError catch (e) { }  /// 07: ok
 
   x is double;  /// 10: ok
   if (x is double) { }  /// 11: ok
   try { x as double; } on CastError catch (e) { }  /// 12: ok
   try { var z = x as double; } on CastError catch (e) { }  /// 13: ok
   y is int;  /// 14: ok
   if (y is int) { }  /// 15: ok
   try { y as int; } on CastError catch (e) { }  /// 16: ok
   try { var z = y as int; } on CastError catch (e) { }  /// 17: ok
 
+  // It is a compile-time error if evaluation of a constant object results in
+  // an uncaught exception being thrown, a JavascriptCompatibilityError here.
   "${1.0}";  /// 20: compile-time error
   var z = "${1.0}";  /// 21: compile-time error
+
   (1.0).toString();  /// 22: ok
   var z = (1.0).toString();  /// 23: ok
   "$y";  /// 24: ok
   var z = "$y";  /// 25: ok
   y.toString();  /// 26: ok
   var z = y.toString();  /// 27: ok
 
   var a = "yz";
   var b = "xyz";
   b = b.substring(1);
   if (identical(a, b)) { }  /// 28: ok
 
   if (identical(x, y)) { }  /// 29: ok
   if (identical(y, x)) { }  /// 30: ok
 
   if (x > 10) {
     // Optimized code.
     x is double;  /// 40: ok
     if (x is double) { }  /// 41: ok
     try { x as double; } on CastError catch (e) { }  /// 42: ok
     try { var z = x as double; } on CastError catch (e) { }  /// 43: ok
     y is int;  /// 44: ok
     if (y is int) { }  /// 45: ok
     try { y as int; } on CastError catch (e) { }  /// 46: ok
     try { var z = y as int; } on CastError catch (e) { }  /// 47: ok
 
     "${1.0}";  /// 50: compile-time error
     var z = "${1.0}";  /// 51: compile-time error
+
     (1.0).toString();  /// 52: ok
     var z = (1.0).toString();  /// 53: ok
     "$y";  /// 54: ok
     var z = "$y";  /// 55: ok
     y.toString();  /// 56: ok
     var z = y.toString();  /// 57: ok
 
     var a = "yz";
     var b = "xyz";
     b = b.substring(1);
@@ skipping 101 matching lines @@
     }
   }
   Expect.equals(1, numWarnings);
   // No warnings (errors) should be issued after this point.
   for (var i = 0; i < 20; i++) {
     k(i * 1.0, i);
     k(i * 1.0, i + 0.5);
   }
 }