Make dart2js String.split match the VM version.

sdk/lib/_internal/compiler/js_lib/regexp_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.
 
 part of _js_helper;
 
 // Helper method used by internal libraries.
 regExpGetNative(JSSyntaxRegExp regexp) => regexp._nativeRegExp;
 
 /**
  * Returns a native version of the RegExp with the global flag set.
  *
  * The RegExp's `lastIndex` property is zero when it is returned.
  *
  * The returned regexp is shared, and its `lastIndex` property may be
  * modified by other uses, so the returned regexp must be used immediately
  * when it's returned, with no user-provided code run in between.
  */
 regExpGetGlobalNative(JSSyntaxRegExp regexp) {
   var nativeRegexp = regexp._nativeGlobalVersion;
   JS("void", "#.lastIndex = 0", nativeRegexp);
   return nativeRegexp;
 }
 
+/**
+ * Computes the number of captures in a regexp.
+ *
+ * This currently involves creating a new RegExp object with a different
+ * source and running it against the empty string (the last part is usually
+ * fast).
+ *
+ * The JSSyntaxRegExp could cache the result, and set the cache any time
+ * it finds a match.
+ */
+int regExpCaptureCount(JSSyntaxRegExp regexp) {
+  var nativeAnchoredRegExp = regexp._nativeAnchoredVersion;
+  var match = JS('JSExtendableArray', "#.exec('')", nativeAnchoredRegExp);
+  // The native-anchored regexp always have one capture more than the original,
+  // and always matches the empty string.
+  return match.length - 2;
+}
+
 class JSSyntaxRegExp implements RegExp {
   final String pattern;
   final _nativeRegExp;
   var _nativeGlobalRegExp;
   var _nativeAnchoredRegExp;
 
+  String toString() => "RegExp/$pattern/";
 
   JSSyntaxRegExp(String source,
                  { bool multiLine: false,
                    bool caseSensitive: true })
       : this.pattern = source,
         this._nativeRegExp =
             makeNative(source, multiLine, caseSensitive, false);
 
   get _nativeGlobalVersion {
     if (_nativeGlobalRegExp != null) return _nativeGlobalRegExp;
@@ skipping 168 matching lines @@
     _current = null;
     _string = null;  // Marks iteration as ended.
     return false;
   }
 }
 
 /** Find the first match of [regExp] in [string] at or after [start]. */
 Match firstMatchAfter(JSSyntaxRegExp regExp, String string, int start) {
   return regExp._execGlobal(string, start);
 }