Improve performance of String.split for 1 and 0 character results.

runtime/lib/string_base.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.
 
 /**
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
 class _StringBase {
 
@@ skipping 134 matching lines @@
 
     if ((startIndex < 0) || (startIndex > this.length)) {
       throw new RangeError.value(startIndex);
     }
     if ((endIndex < 0) || (endIndex > this.length)) {
       throw new RangeError.value(endIndex);
     }
     if (startIndex > endIndex) {
       throw new RangeError.value(startIndex);
     }
+    if (startIndex == endIndex) {
+      return "";
+    }
+    if ((startIndex + 1) == endIndex) {
+      return this[startIndex];
+    }
     return _substringUnchecked(startIndex, endIndex);
   }
 
   String _substringUnchecked(int startIndex, int endIndex)
       native "StringBase_substringUnchecked";
 
   String trim() {
     final int len = this.length;
     int first = 0;
     for (; first < len; first++) {
@@ skipping 313 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int start;
   final String str;
   final String pattern;
 }