Some kernel->ssa cleanups.

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

Index: pkg/compiler/lib/src/ssa/graph_builder.dart
diff --git a/pkg/compiler/lib/src/ssa/graph_builder.dart b/pkg/compiler/lib/src/ssa/graph_builder.dart
index 9898c6a96212c2b817c851af41b88b54a2160bbc..a6ef442029211640d91a884b05271e4dfd607ca0 100644
--- a/pkg/compiler/lib/src/ssa/graph_builder.dart
+++ b/pkg/compiler/lib/src/ssa/graph_builder.dart
@@ -4,9 +4,9 @@
 
 import '../compiler.dart';
 import '../elements/elements.dart';
+import '../js_backend/js_backend.dart';
 import '../resolution/tree_elements.dart';
 import '../types/types.dart';
-
 import 'jump_handler.dart';
 import 'locals_handler.dart';
 import 'nodes.dart';
@@ -23,6 +23,9 @@ abstract class GraphBuilder {
   /// A reference to the compiler.
   Compiler compiler;
 
+  /// The JavaScript backend we are targeting in this compilation.
+  JavaScriptBackend get backend;
+
   /// The tree elements for the element being built into an SSA graph.
   TreeElements get elements;
 
@@ -58,7 +61,10 @@ abstract class GraphBuilder {
   HInstruction popBoolified();
 
   /// Pushes a boolean checking [expression] against null.
-  pushCheckNull(HInstruction expression);
+  pushCheckNull(HInstruction expression) {
+    push(new HIdentity(
+        expression, graph.addConstantNull(compiler), null, backend.boolType));
+  }
 
   void dup() {
     stack.add(stack.last);