Add check for duplicate field initialization

lib/compiler/implementation/universe.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.
 
 class Universe {
   Map<Element, CodeBuffer> generatedCode;
   Map<Element, CodeBuffer> generatedBailoutCode;
   final Set<ClassElement> instantiatedClasses;
   final Set<SourceString> instantiatedClassInstanceFields;
   final Set<FunctionElement> staticFunctionsNeedingGetter;
@@ skipping 68 matching lines @@
   static final SelectorKind OPERATOR = const SelectorKind('operator');
   static final SelectorKind INDEX = const SelectorKind('index');
 
   toString() => name;
 }
 
 class Selector implements Hashable {
   // The numbers of arguments of the selector. Includes named arguments.
   final int argumentCount;
   final SelectorKind kind;
-  final List<SourceString> namedArguments = const <SourceString>[];
-  final List<SourceString> orderedNamedArguments = const <SourceString>[];
+  final List<SourceString> namedArguments;
+  final List<SourceString> orderedNamedArguments;
 
   // The const constructor.
-  const Selector.constant(this.kind, this.argumentCount);
+  const Selector.constant(this.kind, this.argumentCount)
+    : this.namedArguments = const <SourceString>[],
+      this.orderedNamedArguments = const <SourceString>[];
 
   Selector(
       this.kind,
       this.argumentCount,
       [List<SourceString> namedArguments = const <SourceString>[]])
     : this.namedArguments = namedArguments,
       this.orderedNamedArguments = namedArguments.isEmpty()
           ? namedArguments
           : <SourceString>[];
 
@@ skipping 241 matching lines @@
 
     return false;
   }
 
   bool operator ==(other) {
     if (other is !TypedSelector) return false;
     if (other.receiverType !== receiverType) return false;
     return super == other;
   }
 }