Implement Double.{toString, toStringAsExponential, toStringAsPrecision}

runtime/lib/double.dart

Index: runtime/lib/double.dart
diff --git a/runtime/lib/double.dart b/runtime/lib/double.dart
index 65dd3e779fe09554defd0d6ad7816fe5aedd4db6..c657f14f8139607c2d966db130ce40f55cdf1825 100644
--- a/runtime/lib/double.dart
+++ b/runtime/lib/double.dart
@@ -150,11 +150,20 @@ class Double implements double {
     // look at the fractionDigits first.
 
     // Step 7.
-    if (fractionDigits < 0 || fractionDigits > 20) {
+    if (fractionDigits !== null &&
+        (fractionDigits < 0 || fractionDigits > 20)) {
       // TODO(antonm): should be proper RangeError or Dart counterpart.
       throw "Range error";
     }
 
+    if (isNaN()) return "NaN";
+    if (this == double.INFINITY) return "Infinity";
+    if (this == -double.INFINITY) return "-Infinity";
+
+    // The dart function prints the shortest representation when fractionDigits
+    // equals null. The native function wants -1 instead.
+    fractionDigits = (fractionDigits === null) ? -1 : fractionDigits;
+
     return _toStringAsExponential(fractionDigits);
   }
   String _toStringAsExponential(int fractionDigits)
@@ -173,6 +182,10 @@ class Double implements double {
       throw "Range error";
     }
 
+    if (isNaN()) return "NaN";
+    if (this == double.INFINITY) return "Infinity";
+    if (this == -double.INFINITY) return "-Infinity";
+
     return _toStringAsPrecision(precision);
   }
   String _toStringAsPrecision(int fractionDigits)