Move error checking from scope creation to error reporter (take 2)

pkg/analyzer/lib/src/dart/resolver/scope.dart

 // Copyright (c) 2014, 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 analyzer.src.dart.resolver.scope;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/element/element.dart';
@@ skipping 1089 matching lines @@
   /**
    * Add the given [element] to this scope. If there is already an element with
    * the given name defined in this scope, then an error will be generated and
    * the original element will continue to be mapped to the name. If there is an
    * element with the given name in an enclosing scope, then a warning will be
    * generated but the given element will hide the inherited element.
    */
   void define(Element element) {
     String name = _getName(element);
     if (name != null && !name.isEmpty) {
-      if (_definedNames != null && _definedNames.containsKey(name)) {
-        errorListener
-            .onError(getErrorForDuplicate(_definedNames[name], element));
-      } else {
-        _definedNames ??= new HashMap<String, Element>();
-        _definedNames[name] = element;
-      }
+      _definedNames ??= new HashMap<String, Element>();
+      _definedNames.putIfAbsent(name, () => element);
     }
   }
 
   /**
    * Add the given [element] to this scope without checking for duplication or
    * hiding.
    */
   void defineNameWithoutChecking(String name, Element element) {
     _definedNames ??= new HashMap<String, Element>();
     _definedNames[name] = element;
@@ skipping 143 matching lines @@
 
   /**
    * Define the type parameters declared by the [classElement].
    */
   void _defineTypeParameters(ClassElement classElement) {
     for (TypeParameterElement typeParameter in classElement.typeParameters) {
       define(typeParameter);
     }
   }
 }