Recognise x.==(null) and generate HIdentical

pkg/compiler/lib/src/ssa/builder_kernel.dart

 // Copyright (c) 2016, 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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry, CodegenWorkItem;
 import '../common/names.dart';
 import '../common/tasks.dart' show CompilerTask;
@@ skipping 1480 matching lines @@
       push(new HInvokeDynamicSetter(selector, mask, null, inputs, type));
     } else {
       push(new HInvokeDynamicMethod(
           selector, mask, inputs, type, isIntercepted));
     }
   }
 
   // TODO(het): Decide when to inline
   @override
   void visitMethodInvocation(ir.MethodInvocation invocation) {
+    // Handle `x == null` specially.

[Emily Fortuna, 2016/11/29 20:57:45]

can you elaborate in this comment why x == null is special? 

[sra1, 2016/11/29 23:07:18]

Done.

+    if (invocation.name.name == '==') {
+      ir.Arguments arguments = invocation.arguments;
+      if (arguments.types.isEmpty &&
+          arguments.positional.length == 1 &&
+          arguments.named.isEmpty) {
+        void finishCheckNull(ir.Expression comparand) {
+          comparand.accept(this);
+          pushCheckNull(pop());
+        }
+
+        ir.Expression receiver = invocation.receiver;
+        ir.Expression argument = arguments.positional.first;
+        if (argument is ir.NullLiteral) {
+          finishCheckNull(receiver);
+          return;
+        }
+        if (receiver is ir.NullLiteral) {
+          finishCheckNull(argument);
+          return;
+        }
+      }

[Emily Fortuna, 2016/11/29 20:57:45]

consider making this code block its own function just because it's
self-contained for a specific case. (or not, your preference)

[sra1, 2016/11/29 23:07:18]

Done.

+    }
+
     invocation.receiver.accept(this);
     HInstruction receiver = pop();
 
     _pushDynamicInvocation(
         invocation,
         astAdapter.typeOfInvocation(invocation),
         <HInstruction>[receiver]
           ..addAll(_visitArguments(invocation.arguments)));
   }
 
@@ skipping 87 matching lines @@
     push(new HNot(popBoolified(), backend.boolType));
   }
 
   @override
   void visitStringConcatenation(ir.StringConcatenation stringConcat) {
     KernelStringBuilder stringBuilder = new KernelStringBuilder(this);
     stringConcat.accept(stringBuilder);
     stack.add(stringBuilder.result);
   }
 }