Remove special casing of == null

tests/compiler/dart2js/kernel/compile_from_dill_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Partial test that the closed world computed from [WorldImpact]s derived from
 // kernel is equivalent to the original computed from resolution.
 library dart2js.kernel.compile_from_dill_test;
 
 import 'dart:async';
 import 'dart:io';
@@ skipping 31 matching lines @@
 }
 main() {
   foo();
   bar(true);
   [];
   {};
   new Object();
   new Class('');
   Class.staticField;
   var x = null;
+  var y1 = x == null;
+  var y2 = null == x;
+  var z1 = x?.toString();
+  var z2 = x ?? y1;
+  var z3 = x ??= y2;
+  var w = x == null ? null : x.toString();
   for (int i = 0; i < 10; i++) {
     x = i;
     if (i == 5) break;
   }
+  print(x);
   return x;
 }
 '''
 };
 
 main(List<String> args) {
   asyncTest(() async {
     await mainInternal(args);
   });
 }
@@ skipping 127 matching lines @@
     }
     Map<String, BufferedOutputSink> map2 = collector2.outputMap[outputType];
     checkSets(map1.keys, map2.keys, 'output', equality);
     map1.forEach((String name, BufferedOutputSink output1) {
       BufferedOutputSink output2 = map2[name];
       Expect.stringEquals(output1.text, output2.text);
     });
   });
   return ResultKind.success;
 }