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

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index 7c242ef95659a9872e9810492060f0d695f8dc13..6e7603515e58bfdab4604e7ce14912e816d662a4 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -231,31 +231,7 @@ class _Smi extends _IntegerImplementation implements int {
   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.