a === b -> identical(a, b)

runtime/lib/double.dart

Index: runtime/lib/double.dart
diff --git a/runtime/lib/double.dart b/runtime/lib/double.dart
index 75de3c1149df50d5e7f52922b81e49407f428504..f4025f3b745ed90d6c66233c91568357720d57a6 100644
--- a/runtime/lib/double.dart
+++ b/runtime/lib/double.dart
@@ -157,7 +157,7 @@ class _Double implements double {
     // look at the fractionDigits first.
 
     // Step 7.
-    if (fractionDigits !== null &&
+    if (fractionDigits != null &&
         (fractionDigits < 0 || fractionDigits > 20)) {
       // TODO(antonm): should be proper RangeError or Dart counterpart.
       throw "Range error";
@@ -169,7 +169,7 @@ class _Double implements double {
 
     // The dart function prints the shortest representation when fractionDigits
     // equals null. The native function wants -1 instead.
-    fractionDigits = (fractionDigits === null) ? -1 : fractionDigits;
+    fractionDigits = (fractionDigits == null) ? -1 : fractionDigits;
 
     return _toStringAsExponential(fractionDigits);
   }