dart2js: replace null in arrays with array holes in type representations.

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

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index 4e8e3688ed107a195ebc13f246a13c34f212cab5..7d4314579a2a65cb95ffd3abe9dd4e7dee4a8f9e 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -663,7 +663,12 @@ class TypeRepresentationGenerator extends DartTypeVisitor {
       index++;
     }
     for (DartType type in types) {
-      elements.add(visit(type));
+      jsAst.Expression element = visit(type);
+      if (element is jsAst.LiteralNull) {
+        elements.add(new jsAst.ArrayHole());
+      } else {
+        elements.add(element);
+      }
     }
     return new jsAst.ArrayInitializer(elements);
   }