Minor tweaks to the 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');
 
 #import('coreimpl.dart');
 
 #source('constant_map.dart');
 #source('date_helper.dart');
@@ skipping 577 matching lines @@
 
 builtin$getRange$2(receiver, start, length) {
   if (!isJsArray(receiver)) {
     return UNINTERCEPTED(receiver.getRange(start, length));
   }
   if (0 === length) return [];
   checkNull(start); // TODO(ahe): This is not specified but co19 tests it.
   checkNull(length); // TODO(ahe): This is not specified but co19 tests it.
   if (start is !int) throw new IllegalArgumentException(start);
   if (length is !int) throw new IllegalArgumentException(length);
+  if (length < 0) throw new IllegalArgumentException(length);
   if (start < 0) throw new IndexOutOfRangeException(start);
-  if (length < 0) throw new IllegalArgumentException(length);
   var end = start + length;
   if (end > receiver.length) {
     throw new IndexOutOfRangeException(length);
   }
   if (length < 0) throw new IllegalArgumentException(length);
   return JS('Object', @'#.slice(#, #)', receiver, start, end);
 }
 
 builtin$indexOf$1(receiver, element) {
   if (isJsArray(receiver) || receiver is String) {
@@ skipping 80 matching lines @@
 
 builtin$lastIndexOf$2(receiver, element, start) {
   if (isJsArray(receiver)) {
     return Arrays.lastIndexOf(receiver, element, start);
   } else if (receiver is String) {
     checkNull(element);
     if (element is !String) throw new IllegalArgumentException(element);
     if (start !== null) {
       if (start is !num) throw new IllegalArgumentException(start);
       if (start < 0) return -1;
-      if (start >= receiver.length) start = receiver.length - 1;
+      if (start >= receiver.length) {
+        if (element == "") return receiver.length;
+        start = receiver.length - 1;
+      }
     }
     return stringLastIndexOfUnchecked(receiver, element, start);
   }
   return UNINTERCEPTED(receiver.lastIndexOf(element, start));
 }
 
 stringLastIndexOfUnchecked(receiver, element, start)
   => JS('int', @'#.lastIndexOf(#, #)', receiver, element, start);
 
 builtin$removeRange$2(receiver, start, length) {
@@ skipping 645 matching lines @@
 }
 
 /**
  * Represents the type of Null. The compiler treats this specially.
  */
 class Null {
   factory Null() {
     throw UnsupportedOperationException('new Null()');
   }
 }