Fix 'is' for null.

dart/frog/leg/ssa/codegen.dart

 // Copyright (c) 2012, 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.
 
 class SsaCodeGeneratorTask extends CompilerTask {
   SsaCodeGeneratorTask(Compiler compiler) : super(compiler);
   String get name() => 'SSA code generator';
 
   String generate(WorkItem work, HGraph graph) {
     return measure(() {
@@ skipping 796 matching lines @@
     use(input);
     buffer.add(" $cmp 'string'");
   }
 
   void checkBool(HInstruction input, String cmp) {
     buffer.add('typeof ');
     use(input);
     buffer.add(" $cmp 'boolean'");
   }
 
+  void checkObject(HInstruction input, String cmp) {
+    buffer.add('typeof ');
+    use(input);
+    buffer.add(" $cmp 'object'");
+  }
+
   void checkArray(HInstruction input, String cmp) {
     use(input);
     buffer.add('.constructor $cmp Array');
   }
 
   void visitIs(HIs node) {
     ClassElement element = node.typeExpression;
     LibraryElement coreLibrary = compiler.coreLibrary;
     HInstruction input = node.expression;
     buffer.add('(');
     if (element == coreLibrary.find(const SourceString('Object'))) {
       // TODO(ahe): This probably belongs in the constant folder.
       buffer.add('true');
     } else if (element == coreLibrary.find(const SourceString('String'))) {
       checkString(input, '===');
     } else if (element == coreLibrary.find(const SourceString('double'))) {
       checkDouble(input, '===');
     } else if (element == coreLibrary.find(const SourceString('num'))) {
       checkNum(input, '===');
     } else if (element == coreLibrary.find(const SourceString('bool'))) {
       checkBool(input, '===');
     } else if (element == coreLibrary.find(const SourceString('int'))) {
       checkNum(input, '===');
       buffer.add(' && ');
       checkInt(input, '===');
     } else {
+      checkObject(input, '===');
+      buffer.add(' && (');
       if (element == coreLibrary.find(const SourceString('List'))) {
         checkArray(input, '===');
         buffer.add(' || ');
       }
       buffer.add('!!(');
       use(input);
       buffer.add('.');
       buffer.add(compiler.namer.operatorIs(node.typeExpression));
-      buffer.add(')');
+      buffer.add('))');
     }
     buffer.add(')');
   }
 }
 
 class SsaOptimizedCodeGenerator extends SsaCodeGenerator {
   final List<HTypeGuard> guards;
   int state = 0;
 
   SsaOptimizedCodeGenerator(compiler, work, buffer, parameters, parameterNames)
@@ skipping 330 matching lines @@
       startBailoutSwitch();
     }
   }
 
   void endElse(HIf node) {
     if (node.elseBlock.hasBailouts()) {
       endBailoutSwitch();
     }
   }
 }