Revert "Remove NullPointerException."

pkg/fixnum/test/int_64_test.dart

Index: pkg/fixnum/test/int_64_test.dart
diff --git a/pkg/fixnum/test/int_64_test.dart b/pkg/fixnum/test/int_64_test.dart
index f41f45c78999d098eceb9756c1dff1cd14b93d0b..65d47b3542d796511e141695b19e38b27c63459e 100644
--- a/pkg/fixnum/test/int_64_test.dart
+++ b/pkg/fixnum/test/int_64_test.dart
@@ -145,26 +145,26 @@ void testComparisons() {
 
   try {
     new int64.fromInt(17) < null;
-    Expect.fail("x < null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x < null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   try {
     new int64.fromInt(17) <= null;
-    Expect.fail("x <= null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x <= null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   try {
     new int64.fromInt(17) > null;
-    Expect.fail("x > null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x > null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   try {
     new int64.fromInt(17) < null;
-    Expect.fail("x >= null should throw ArgumentError");
-  } on ArgumentError catch (e) {
+    Expect.fail("x >= null should throw NullPointerException");
+  } on NullPointerException catch (e) {
   }
 
   Expect.isFalse(new int64.fromInt(17) == null);