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

sdk/lib/_internal/compiler/implementation/elements/elements.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.
 
 library elements;
 
 import 'dart:uri';
 
 import 'modelx.dart';
 import '../tree/tree.dart';
@@ skipping 638 matching lines @@
 }
 
 abstract class FunctionSignature {
   DartType get returnType;
   Link<Element> get requiredParameters;
   Link<Element> get optionalParameters;
 
   int get requiredParameterCount;
   int get optionalParameterCount;
   bool get optionalParametersAreNamed;
+  Element get firstOptionalParameter;
 
   int get parameterCount;
   List<Element> get orderedOptionalParameters;
 
   void forEachParameter(void function(Element parameter));
   void forEachRequiredParameter(void function(Element parameter));
   void forEachOptionalParameter(void function(Element parameter));
 
   void orderedForEachParameter(void function(Element parameter));
 }
@@ skipping 189 matching lines @@
   int get resolutionState;
   Token get beginToken;
   Token get endToken;
 
   // TODO(kasperl): Try to get rid of these.
   void set annotatedElement(Element value);
   void set resolutionState(int value);
 
   MetadataAnnotation ensureResolved(Compiler compiler);
 }