Make smi.toString() do its work faster using native string helpers.

runtime/lib/integers.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.
 
 // TODO(srdjan): fix limitations.
 // - shift amount must be a Smi.
 class _IntegerImplementation {
   factory _IntegerImplementation._uninstantiable() {
     throw new UnsupportedError(
         "_IntegerImplementation can only be allocated by the VM");
@@ skipping 213 matching lines @@
   }
   int get hashCode {
     return this;
   }
   int operator ~() native "Smi_bitNegate";
   int get bitLength native "Smi_bitLength";
 
   int _shrFromInt(int other) native "Smi_shrFromInt";
   int _shlFromInt(int other) native "Smi_shlFromInt";
 
-  String toString() {
-    if (this == 0) return "0";
-    var reversed = new List();
-    var val = this < 0 ? -this : this;
-    while (val > 0) {
-      reversed.add((val % 10) + 0x30);
-      val = val ~/ 10;
-    }
-    final int numDigits = reversed.length;
-    List digits;
-    int i;
-    if (this < 0) {
-      digits = new List(numDigits + 1);
-      digits[0] = 0x2D;  // '-'.
-      i = 1;
-    } else {
-      digits = new List(numDigits);
-      i = 0;
-    }
-    int ri = reversed.length - 1;
-    for (; i < digits.length; i++, ri--) {
-      digits[i] = reversed[ri];
-    }
-    return _StringBase.createFromCharCodes(digits);
-  }
+  String toString() native "Smi_toString";
 }
 
 // Represents integers that cannot be represented by Smi but fit into 64bits.
 class _Mint extends _IntegerImplementation implements int {
   factory _Mint._uninstantiable() {
     throw new UnsupportedError(
         "_Mint can only be allocated by the VM");
   }
   int get hashCode {
     return this;
@@ skipping 32 matching lines @@
     } else {
       return 0;
     }
   }
   int _shlFromInt(int other) native "Bigint_shlFromInt";
 
   int pow(int exponent) {
     throw "Bigint.pow not implemented";
   }
 }