Fix crash on mandatory parameters with initializers.

lib/kernel_visitor.dart

 // Copyright (c) 2016, 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.md file.
 
 library rasta.kernel_visitor;
 
 import 'package:kernel/ast.dart' as ir;
 
 import 'package:kernel/accessors.dart' show
     Accessor,
@@ skipping 2066 matching lines @@
       requiredParameterCount = signature.requiredParameterCount;
       signature.forEachParameter((ParameterElement parameter) {
         ir.VariableDeclaration variable = getLocal(parameter);
         if (parameter.isNamed) {
           namedParameters.add(variable);
         } else {
           positionalParameters.add(variable);
         }
       });
       signature.forEachParameter((ParameterElement parameter) {
+        if (!parameter.isOptional) return;
         ir.Expression initializer = parameter.initializer?.accept(this);
         ir.VariableDeclaration variable = getLocal(parameter);
         if (initializer != null) {
           variable.initializer = initializer;
           initializer.parent = variable;
         }
       });
       returnType = typeToIrHack(signature.type.returnType);
     }
     ir.AsyncMarker asyncMarker = ir.AsyncMarker.Sync;
@@ skipping 938 matching lines @@
       : this(null, true, node, initializers);
 
   accept(ir.Visitor v) => throw "unsupported";
 
   visitChildren(ir.Visitor v) => throw "unsupported";
 
   String toString() {
     return "IrFunction($kind, $isConstructor, $node, $initializers)";
   }
 }