Avoid unnecessary copying when parsing doubles.

runtime/lib/convert_patch.dart

 // Copyright (c) 2013, 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.
 
 import "dart:typed_data";
 
 // JSON conversion.
 
 patch _parseJson(String json, reviver(var key, var value)) {
   _BuildJsonListener listener;
@@ skipping 540 matching lines @@
           position++;
           if (position == length) break parsing;
           char = source.codeUnitAt(position);
         } while (CHAR_0 <= char && char <= CHAR_9);
       }
     }
     if (!isDouble) {
       listener.handleNumber(intSign * intValue);
       return position;
     }
-    // Consider whether we can have an int/double.parse that works on part of
-    // a string, to avoid creating the substring.
-    String literal = source.substring(start, position);
     // This correctly creates -0.0 for doubles.
-    listener.handleNumber(double.parse(literal));
+    listener.handleNumber(_parseDouble(source, start, position));
     return position;
   }
 
+  static double _parseDouble(String str,
+                             int start, int end) native "Double_parse";
+
   void fail(int position, [String message]) {
     if (message == null) message = "Unexpected character";
     listener.fail(source, position, message);
     // If the listener didn't throw, do it here.
     String slice;
     int sliceEnd = position + 20;
     if (sliceEnd > source.length) {
       slice = "'${source.substring(position)}'";
     } else {
       slice = "'${source.substring(position, sliceEnd)}...'";
     }
     throw new FormatException("Unexpected character at $position: $slice");
   }
 }
 
 // UTF-8 conversion.
 
 patch class _Utf8Encoder {
   /* patch */ static List<int> _createBuffer(int size) => new Uint8List(size);
 }