Don't make defensive copies of 'definedNames'.

pkg/analyzer/lib/src/generated/resolver.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 engine.resolver;
 
 import 'dart:collection';
 
 import 'ast.dart';
 import 'constant.dart';
@@ skipping 9243 matching lines @@
    * @param definedNames the mapping from names that are defined in this namespace to the
    *          corresponding elements
    */
   Namespace(this._definedNames);
 
   /**
    * Return a table containing the same mappings as those defined by this namespace.
    *
    * @return a table containing the same mappings as those defined by this namespace
    */
-  Map<String, Element> get definedNames =>
-      new HashMap<String, Element>.from(_definedNames);
+  Map<String, Element> get definedNames => _definedNames;
 
   /**
    * Return the element in this namespace that is available to the containing scope using the given
    * name.
    *
    * @param name the name used to reference the
    * @return the element represented by the given identifier
    */
   Element get(String name) => _definedNames[name];
 }
@@ skipping 129 matching lines @@
    * Apply the given combinators to all of the names in the given mapping table.
    *
    * @param definedNames the mapping table to which the namespace operations are to be applied
    * @param combinators the combinators to be applied
    */
   HashMap<String, Element> _applyCombinators(
       HashMap<String, Element> definedNames,
       List<NamespaceCombinator> combinators) {
     for (NamespaceCombinator combinator in combinators) {
       if (combinator is HideElementCombinator) {
-        _hide(definedNames, combinator.hiddenNames);
+        definedNames = _hide(definedNames, combinator.hiddenNames);
       } else if (combinator is ShowElementCombinator) {
         definedNames = _show(definedNames, combinator.shownNames);
       } else {
         // Internal error.
         AnalysisEngine.instance.logger
             .logError("Unknown type of combinator: ${combinator.runtimeType}");
       }
     }
     return definedNames;
   }
@@ skipping 58 matching lines @@
           definedNames,
           (library.context as InternalAnalysisContext)
               .getPublicNamespace(library));
       return definedNames;
     } finally {
       visitedElements.remove(library);
     }
   }
 
   /**
-   * Hide all of the given names by removing them from the given collection of defined names.
-   *
-   * @param definedNames the names that were defined before this operation
-   * @param hiddenNames the names to be hidden
+   * Return a new map of names which has all the names from [definedNames]
+   * with exception of [hiddenNames].
    */
-  void _hide(HashMap<String, Element> definedNames, List<String> hiddenNames) {
+  Map<String, Element> _hide(
+      HashMap<String, Element> definedNames, List<String> hiddenNames) {
+    HashMap<String, Element> newNames =
+        new HashMap<String, Element>.from(definedNames);
     for (String name in hiddenNames) {
-      definedNames.remove(name);
-      definedNames.remove("$name=");
+      newNames.remove(name);
+      newNames.remove("$name=");
     }
+    return newNames;
   }
 
   /**
-   * Show only the given names by removing all other names from the given collection of defined
-   * names.
-   *
-   * @param definedNames the names that were defined before this operation
-   * @param shownNames the names to be shown
+   * Return a new map of names which has only [shownNames] from [definedNames].
    */
   HashMap<String, Element> _show(
       HashMap<String, Element> definedNames, List<String> shownNames) {
     HashMap<String, Element> newNames = new HashMap<String, Element>();
     for (String name in shownNames) {
       Element element = definedNames[name];
       if (element != null) {
         newNames[name] = element;
       }
       String setterName = "$name=";
@@ skipping 6427 matching lines @@
     nonFields.add(node);
     return null;
   }
 
   @override
   Object visitNode(AstNode node) => node.accept(TypeResolverVisitor_this);
 
   @override
   Object visitWithClause(WithClause node) => null;
 }