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

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

Index: pkg/analyzer/lib/src/dart/resolver/scope.dart
diff --git a/pkg/analyzer/lib/src/dart/resolver/scope.dart b/pkg/analyzer/lib/src/dart/resolver/scope.dart
index ba5b275cbb1395f58ec9433ece3449d9870c7cfa..b514b224044e57f52856c8e7350f225862c413de 100644
--- a/pkg/analyzer/lib/src/dart/resolver/scope.dart
+++ b/pkg/analyzer/lib/src/dart/resolver/scope.dart
@@ -1107,13 +1107,8 @@ abstract class Scope {
   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);
     }
   }