Improve performance of String.fromCharCodes by implementing it in Dart. Add tow internal natives to…

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);
   }
 }
 
 
 /**
  * [_StringBase] contains common methods used by concrete String
  * implementations, e.g., _OneByteString.
  */
 class _StringBase {
 
   factory _StringBase._uninstantiable() {
     throw new UnsupportedError(
         "_StringBase can't be instaniated");
   }
 
   int get hashCode native "String_getHashCode";
 
   /**
    *  Create the most efficient string representation for specified
    *  [codePoints].
    */
   static String createFromCharCodes(Iterable<int> charCodes) {
-    // TODO(srdjan): Also skip copying of typed arrays.
-    if (charCodes is! _ObjectArray &&
-        charCodes is! _GrowableObjectArray &&
-        charCodes is! _ImmutableArray) {
-      charCodes = new List<int>.from(charCodes, growable: false);
+    if (charCodes != null) {
+      // TODO(srdjan): Also skip copying of typed arrays.
+      if (charCodes is! _ObjectArray &&
+          charCodes is! _GrowableObjectArray &&
+          charCodes is! _ImmutableArray) {
+        charCodes = new List<int>.from(charCodes, growable: false);
+      }
+
+      bool isOneByteString = true;
+      for (int i = 0; i < charCodes.length; i++) {
+        int e = charCodes[i];
+        if (e is! int) throw new ArgumentError(e);
+        // Is e Latin1?
+        if ((e < 0) || (e > 0xFF)) {
+          isOneByteString = false;
+          break;
+        }
+      }
+      if (isOneByteString) {
+        var s = _OneByteString._allocate(charCodes.length);
+        for (int i = 0; i < charCodes.length; i++) {
+          s._setAt(i, charCodes[i]);
+        }
+        return s;
+      }
     }
-
     return _createFromCodePoints(charCodes);
   }
 
   static String _createFromCodePoints(List<int> codePoints)
       native "StringBase_createFromCodePoints";
 
   String operator [](int index) native "String_charAt";
 
   int codeUnitAt(int index) native "String_codeUnitAt";
 
@@ skipping 412 matching lines @@
 
   List<String> _splitWithCharCode(int charCode)
       native "OneByteString_splitWithCharCode";
 
   List<String> split(Pattern pattern) {
     if ((pattern is _OneByteString) && (pattern.length == 1)) {
       return _splitWithCharCode(pattern.codeUnitAt(0));
     }
     return super.split(pattern);
   }
+
+  // Allocates a string of given length, expecting its content to be
+  // set using _setAt.
+  static _OneByteString _allocate(int length) native "OneByteString_allocate";
+
+  // Code point value must be a valid Latin1 (0..0xFF). Index must be valid.
+  void _setAt(int index, int codePoint) native "OneByteString_setAt";
 }
 
 
 class _TwoByteString extends _StringBase implements String {
   factory _TwoByteString._uninstantiable() {
     throw new UnsupportedError(
         "_TwoByteString can only be allocated by the VM");
   }
 
   // Checks for one-byte whitespaces only.
@@ skipping 110 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);
 }