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

runtime/lib/string_base.dart

Index: runtime/lib/string_base.dart
===================================================================
--- runtime/lib/string_base.dart	(revision 15427)
+++ runtime/lib/string_base.dart	(working copy)
@@ -152,6 +152,12 @@
     if (startIndex > endIndex) {
       throw new RangeError.value(startIndex);
     }
+    if (startIndex == endIndex) {
+      return "";
+    }
+    if ((startIndex + 1) == endIndex) {
+      return this[startIndex];
+    }
     return _substringUnchecked(startIndex, endIndex);
   }