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

tests/standalone/javascript_compatibility_errors_test.dart

Index: tests/standalone/javascript_compatibility_errors_test.dart
===================================================================
--- tests/standalone/javascript_compatibility_errors_test.dart	(revision 37011)
+++ tests/standalone/javascript_compatibility_errors_test.dart	(working copy)
@@ -35,6 +35,11 @@
   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
@@ -54,6 +59,11 @@
     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
   }
 }