Add javascript compatibility warnings for strings that are not identical, but

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 code.
@@ skipping 17 matching lines @@
 
   "${1.0}";  /// 20: compile-time error
   var z = "${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 (x > 10) {
     // Optimized code.
     x is double;  /// 30: ok
     if (x is double) { }  /// 31: ok
     try { x as double; } on CastError catch (e) { }  /// 32: ok
     try { var z = x as double; } on CastError catch (e) { }  /// 33: ok
     y is int;  /// 34: ok
     if (y is int) { }  /// 35: ok
     try { y as int; } on CastError catch (e) { }  /// 36: ok
     try { var z = y as int; } on CastError catch (e) { }  /// 37: ok
 
     "${1.0}";  /// 40: compile-time error
     var z = "${1.0}";  /// 41: compile-time error
     (1.0).toString();  /// 42: ok
     var z = (1.0).toString();  /// 43: ok
     "$y";  /// 44: ok
     var z = "$y";  /// 45: ok
     y.toString();  /// 46: ok
     var z = y.toString();  /// 47: ok
+
+    var a = "yz";
+    var b = "xyz";
+    b = b.substring(1);
+    if (identical(a, b)) { }  /// 48: ok
   }
 }
 
 k(x, y) {
   // Unoptimized code.
   1.5 is double;
   if (1.5 is double) { x++; }
   try { 1.5 as double; } on CastError catch (e) { }
   try { var y = 1.5 as double; } on CastError catch (e) { }
   1.5 is int;
@@ skipping 69 matching lines @@
       },
       isJavascriptCompatibilityError);
 
   // 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);
   }
 }