Track argument types, and remove obsolete code that also used to track it.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

 // Copyright (c) 2013, 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 elements.modelx;
 
 import 'dart:uri';
 import 'dart:collection' show LinkedHashMap;
 
 import 'elements.dart';
@@ skipping 1050 matching lines @@
     List<Element> list = new List<Element>.from(optionalParameters);
     if (optionalParametersAreNamed) {
       list.sort((Element a, Element b) {
         return a.name.slowToString().compareTo(b.name.slowToString());
       });
     }
     _orderedOptionalParameters = list;
     return list;
   }
 
+  Element get firstOptionalParameter => optionalParameters.head;
+
   void forEachParameter(void function(Element parameter)) {
     forEachRequiredParameter(function);
     forEachOptionalParameter(function);
   }
 
   void orderedForEachParameter(void function(Element parameter)) {
     forEachRequiredParameter(function);
     orderedOptionalParameters.forEach(function);
   }
 
@@ skipping 935 matching lines @@
 
   MetadataAnnotation ensureResolved(Compiler compiler) {
     if (resolutionState == STATE_NOT_STARTED) {
       compiler.resolver.resolveMetadataAnnotation(this);
     }
     return this;
   }
 
   String toString() => 'MetadataAnnotation($value, $resolutionState)';
 }