Use Expect from core library.

dart/frog/leg/lib/js_helper.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.
 
 #library('js_helper');
 
 $throw(String msg) {
   var e = JS("Object", @"new Error($0)", msg);
   var hasTrace = JS("bool", @"Error.captureStackTrace !== (void 0)");
   if (hasTrace) {
@@ skipping 321 matching lines @@
     if (length == null) return JS("Object", @"new Array()");
     if (!(length is int)) throw "Invalid argument";
     if (length < 0) throw "Negative size";
     return JS("Object", @"new Array($0)", length);
   }
 
   static num dateNow() => JS("num", @"Date.now()");
 
   static num mathFloor(num value) => JS("num", @"Math.floor($0)", value);
 }
+
+builtin$compareTo$1(a, b) {
+  if (checkNumbers(a, b, 'illegal argument')) {
+    if (a < b) {
+      return -1;
+    } else if (a > b) {
+      return 1;
+    } else if (a == b) {
+      if (a == 0) {
+        bool aIsNegative = a.isNegative();
+        bool bIsNegative = b.isNegative();
+        if (aIsNegative == bIsNegative) return 0;
+        if (aIsNegative) return -1;
+        return 1;
+      }
+      return 0;
+    } else if (a.isNaN()) {
+      if (b.isNaN()) {
+        return 0;
+      }
+      return 1;
+    } else {
+      return -1;
+    }
+  } else if (a is String) {
+    throw 'String.compareTo is not implemented';
+  } else {
+    return UNINTERCEPTED(a.compareTo(b));
+  }
+}