dart2js cps: Add instruction for null checks.

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

 // Copyright (c) 2014, 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.
 
 library code_generator;
 
 import 'glue.dart';
 
 import '../../closure.dart' show
     ClosureClassElement;
@@ skipping 927 matching lines @@
   }
 
   @override
   void visitYield(tree_ir.Yield node) {
     js.Expression value = visitExpression(node.input);
     accumulator.add(new js.DartYield(value, node.hasStar));
     visitStatement(node.next);
   }
 
   @override
+  void visitNullCheck(tree_ir.NullCheck node) {
+    js.Expression value = visitExpression(node.value);
+    js.Expression access = node.selector != null
+        ? js.js('#.#', [value, glue.invocationName(node.selector)])
+        : js.js('#.toString', [value]);
+    if (node.condition != null) {
+      js.Expression condition = visitExpression(node.condition);
+      js.Statement body = isNullReturn(node.next)
+          ? new js.ExpressionStatement(access)
+          : new js.Return(access);
+      accumulator.add(new js.If.noElse(condition, body));
+    } else {
+      accumulator.add(new js.ExpressionStatement(access));
+    }
+    visitStatement(node.next);
+  }
+
+  @override
   js.Expression visitApplyBuiltinOperator(tree_ir.ApplyBuiltinOperator node) {
     List<js.Expression> args = visitExpressionList(node.arguments);
     switch (node.operator) {
       case BuiltinOperator.NumAdd:
         return new js.Binary('+', args[0], args[1]);
       case BuiltinOperator.NumSubtract:
         return new js.Binary('-', args[0], args[1]);
       case BuiltinOperator.NumMultiply:
         return new js.Binary('*', args[0], args[1]);
       case BuiltinOperator.NumDivide:
@@ skipping 164 matching lines @@
   void registerDefaultParameterValues(ExecutableElement element) {
     if (element is! FunctionElement) return;
     FunctionElement function = element;
     if (function.isStatic) return; // Defaults are inlined at call sites.
     function.functionSignature.forEachOptionalParameter((param) {
       ConstantValue constant = glue.getDefaultParameterValue(param);
       registry.registerCompileTimeConstant(constant);
     });
   }
 }