Recognise x.==(null) and generate HIdentical

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

Index: pkg/compiler/lib/src/ssa/builder_kernel.dart
diff --git a/pkg/compiler/lib/src/ssa/builder_kernel.dart b/pkg/compiler/lib/src/ssa/builder_kernel.dart
index 46e5002e6ae86963495bff2853fb7693b56f8c43..ef30c202237a5269965c9cfc1b75f8c8e8c988ab 100644
--- a/pkg/compiler/lib/src/ssa/builder_kernel.dart
+++ b/pkg/compiler/lib/src/ssa/builder_kernel.dart
@@ -1498,6 +1498,30 @@ class KernelSsaBuilder extends ir.Visitor with GraphBuilder {
   // 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();