Optimize String.fromCharCode.

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) {
+    if (charCode >= 0) {
+      if (charCode <= 0xff) {
+        return _OneByteString._allocate(1).._setAt(0, charCode);
+      }
+      if (charCode <= 0xffff) {
+        return _StringBase._createFromCodePoints(new _List(1)..[0] = charCode);
+      }
+      if (charCode <= 0x10ffff) {
+        var low = 0xDC00 | (charCode & 0x3ff);
+        int bits = charCode - 0x10000;
+        var high = 0xD800 | (bits >> 10);
+        return  _StringBase._createFromCodePoints(new _List(2)..[0] = high
+                                                              ..[1] = low);
+      }
+    }
+    throw new RangeError.range(charCode, 0, 0x10ffff);
+  }
+
   /* patch */ const factory String.fromEnvironment(String name,
                                                    {String defaultValue})
       native "String_fromEnvironment";
 }
 
 
 /**
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
@@ skipping 923 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);
 }