Serialize ParameterElement.node and ParameterElement.initializer for type inference

pkg/compiler/lib/src/serialization/modelz.dart

 // Copyright (c) 2015, 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.
 
 /// Implementation of the element model used for deserialiation.
 ///
 /// These classes are created by [ElementDeserializer] triggered by the
 /// [Deserializer].
 
 library dart2js.serialization.modelz;
@@ skipping 1849 matching lines @@
   FunctionElement get functionDeclaration {
     if (_functionDeclaration == null) {
       _functionDeclaration = _decoder.getElement(Key.FUNCTION);
     }
     return _functionDeclaration;
   }
 
   @override
   FunctionSignature get functionSignature => _unsupported('functionSignature');
 
-  // TODO(johnniwinther): HACK. Remove [initializer] and [node] on
+  // TODO(johnniwinther): Remove [initializer] and [node] on
   // [ParameterElementZ] when the inference does need these.
   @override
-  Expression get initializer => null;
+  Expression initializer;
 
   @override
-  Node get node => null;
+  Node node;
 
   @override
   bool get isNamed => _decoder.getBool(Key.IS_NAMED);
 
   @override
   bool get isOptional => _decoder.getBool(Key.IS_OPTIONAL);
 
   @override
   LibraryElement get library => executableContext.library;
 
@@ skipping 214 matching lines @@
   }
 
   @override
   ElementKind get kind => ElementKind.PREFIX;
 
   @override
   Element lookupLocalMember(String memberName) {
     return _unsupported('lookupLocalMember');
   }
 }