Fix one missing invalid encoding case.

runtime/vm/object_test.cc

Index: runtime/vm/object_test.cc
===================================================================
--- runtime/vm/object_test.cc	(revision 39145)
+++ runtime/vm/object_test.cc	(working copy)
@@ -430,11 +430,15 @@
 TEST_CASE(StringDecodeIRIInvalid) {
   String& input = String::Handle();
   input = String::New("file%");
-  EXPECT(String::DecodeIRI(input) == String::null());
+  String& decoded = String::Handle();
+  decoded = String::DecodeIRI(input);
+  EXPECT(decoded.IsNull());
   input = String::New("file%3");
-  EXPECT(String::DecodeIRI(input) == String::null());
+  decoded = String::DecodeIRI(input);
+  EXPECT(decoded.IsNull());
   input = String::New("file%3g");
-  EXPECT(String::DecodeIRI(input) == String::null());
+  decoded = String::DecodeIRI(input);
+  EXPECT(decoded.IsNull());
 }