Implement faster splitting with empty string as pattern. Add special native for computing substring…

runtime/lib/string_base.dart

Index: runtime/lib/string_base.dart
===================================================================
--- runtime/lib/string_base.dart	(revision 15545)
+++ runtime/lib/string_base.dart	(working copy)
@@ -269,6 +269,9 @@
   }
 
   List<String> split(Pattern pattern) {
+    if ((pattern is String) && pattern.isEmpty) {
+      return splitChars();
+    }
     int length = this.length;
     Iterator iterator = pattern.allMatches(this).iterator();
     if (length == 0 && iterator.hasNext) {
@@ -375,6 +378,8 @@
       ((9 <= codePoint) && (codePoint <= 13)); // CR, LF, TAB, etc.
   }
 
+  String _substringUnchecked(int startIndex, int endIndex)
+      native "OneByteString_substringUnchecked";
 }