Some kernel->ssa cleanups.

pkg/compiler/lib/src/ssa/graph_builder.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 '../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';
 
 /// Base class for objects that build up an SSA graph.
 ///
 /// This contains helpers for building the graph and tracking information about
 /// the current state of the graph being built.
 abstract class GraphBuilder {
   /// Holds the resulting SSA graph.
   final HGraph graph = new HGraph();
 
   // TODO(het): remove this
   /// 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;
 
   /// Used to track the locals while building the graph.
   LocalsHandler localsHandler;
 
   /// A stack of instructions.
   ///
   /// We build the SSA graph by simulating a stack machine.
   List<HInstruction> stack = <HInstruction>[];
@@ skipping 15 matching lines @@
   HInstruction pop() {
     return stack.removeLast();
   }
 
   /// Pops the most recent instruction from the stack and 'boolifies' it.
   ///
   /// Boolification is checking if the value is '=== true'.
   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);
   }
 
   HBasicBlock _current;
 
   /// The current block to add instructions to. Might be null, if we are
   /// visiting dead code, but see [isReachable].
   HBasicBlock get current => _current;
@@ skipping 84 matching lines @@
   HSubGraphBlockInformation wrapStatementGraph(SubGraph statements) {
     if (statements == null) return null;
     return new HSubGraphBlockInformation(statements);
   }
 
   HSubExpressionBlockInformation wrapExpressionGraph(SubExpression expression) {
     if (expression == null) return null;
     return new HSubExpressionBlockInformation(expression);
   }
 }