Special case substring(0, length) to return itself.

runtime/lib/string_patch.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.
 
 patch class String {
   /* patch */ factory String.fromCharCodes(Iterable<int> charCodes) {
     return _StringBase.createFromCharCodes(charCodes);
   }
 
   /* patch */ factory String.fromCharCode(int charCode) {
@@ skipping 229 matching lines @@
 
   String _substringUnchecked(int startIndex, int endIndex) {
     assert(endIndex != null);
     assert((startIndex >= 0) && (startIndex <= this.length));
     assert((endIndex >= 0) && (endIndex <= this.length));
     assert(startIndex <= endIndex);
 
     if (startIndex == endIndex) {
       return "";
     }
+    if ((startIndex == 0) && (endIndex == this.length)) {
+      return this;
+    }
     if ((startIndex + 1) == endIndex) {
       return this[startIndex];
     }
     return _substringUncheckedNative(startIndex, endIndex);
   }
 
   String _substringUncheckedNative(int startIndex, int endIndex)
       native "StringBase_substringUnchecked";
 
   // Checks for one-byte whitespaces only.
@@ skipping 741 matching lines @@
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }