fix performance of setType

lib/src/compiler/code_generator.dart

 // Copyright (c) 2015, 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.
 
 import 'dart:collection' show HashMap, HashSet;
 import 'dart:math' show min, max;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart' show Token, TokenType;
@@ skipping 679 matching lines @@
 
     var body = <JS.Statement>[];
     var extensions = _extensionsToImplement(classElem);
     _initExtensionSymbols(classElem, methods, fields, body);
     _emitSuperHelperSymbols(_superHelperSymbols, body);
 
     // Emit the class, e.g. `core.Object = class Object { ... }`
     _defineClass(classElem, className, classExpr, body);
 
     // Emit things that come after the ES6 `class ... { ... }`.
-    _setBaseClass(classElem, className, body);
+    var jsPeerName = _getJSPeerName(classElem);
+    _setBaseClass(classElem, className, jsPeerName, body);
     _defineNamedConstructors(ctors, body, className);
     _emitVirtualFieldSymbols(virtualFieldSymbols, body);
     _emitClassSignature(methods, classElem, ctors, extensions, className, body);
     _defineExtensionMembers(extensions, className, body);
     _emitClassMetadata(node.metadata, className, body);
 
     JS.Statement classDef = _statement(body);
     var typeFormals = classElem.typeParameters;
     if (typeFormals.isNotEmpty) {
       classDef = _defineClassTypeArguments(classElem, typeFormals, classDef);
     }
 
     body = <JS.Statement>[classDef];
     _emitStaticFields(staticFields, staticFieldOverrides, classElem, body);
-    _registerExtensionType(classElem, body);
+    _registerExtensionType(classElem, jsPeerName, body);
     return _statement(body);
   }
 
   void _emitSuperHelperSymbols(
       List<JS.TemporaryId> superHelperSymbols, List<JS.Statement> body) {
     for (var id in superHelperSymbols) {
       body.add(js.statement('const # = Symbol(#)', [id, js.string(id.name)]));
     }
     superHelperSymbols.clear();
   }
@@ skipping 408 matching lines @@
         classElem,
         (a) =>
             isJsPeerInterface(a) ||
             isNativeAnnotation(a) && _extensionTypes.isNativeClass(classElem));
     if (jsPeerName != null && jsPeerName.contains(',')) {
       jsPeerName = jsPeerName.split(',')[0];
     }
     return jsPeerName;
   }
 
-  void _registerExtensionType(ClassElement classElem, List<JS.Statement> body) {
-    var jsPeerName = _getJSPeerName(classElem);
+  void _registerExtensionType(
+      ClassElement classElem, String jsPeerName, List<JS.Statement> body) {
     if (jsPeerName != null) {
-      // TODO(jmesserly): this copies the dynamic members.
-      // Probably fine for objects coming from JS, but not if we actually
-      // want to support construction of instances with generic types other
-      // than dynamic. See issue #154 for Array and List<E> related bug.
       body.add(js.statement('dart.registerExtension(dart.global.#, #);',
           [_propertyName(jsPeerName), _emitTopLevelName(classElem)]));
     }
   }
 
   void _setBaseClass(ClassElement classElem, JS.Expression className,
-      List<JS.Statement> body) {
-    String jsPeerName = _getJSPeerName(classElem);
-    JS.Expression newBaseClass;
+      String jsPeerName, List<JS.Statement> body) {
     if (jsPeerName != null && classElem.typeParameters.isNotEmpty) {
       // TODO(jmesserly): we should really just extend Array in the first place.
-      newBaseClass = js.call('dart.global.#', [jsPeerName]);
+      var newBaseClass = js.call('dart.global.#', [jsPeerName]);
+      body.add(js.statement(
+          'dart.setExtensionBaseClass(#, #);', [className, newBaseClass]));
     } else if (_hasDeferredSupertype.contains(classElem)) {
-      newBaseClass = _emitType(classElem.type.superclass,
+      var newBaseClass = _emitType(classElem.type.superclass,
           subClass: classElem, className: className);
-    }
-    if (newBaseClass != null) {
       body.add(
           js.statement('dart.setBaseClass(#, #);', [className, newBaseClass]));
     }
   }
 
   void _defineNamedConstructors(List<ConstructorDeclaration> ctors,
       List<JS.Statement> body, JS.Expression className) {
     for (ConstructorDeclaration member in ctors) {
       if (member.name != null && member.factoryKeyword == null) {
         body.add(js.statement('dart.defineNamedConstructor(#, #);',
@@ skipping 3379 matching lines @@
 }
 
 bool isLibraryPrefix(Expression node) =>
     node is SimpleIdentifier && node.staticElement is PrefixElement;
 
 LibraryElement _getLibrary(AnalysisContext c, String uri) =>
     c.computeLibraryElement(c.sourceFactory.forUri(uri));
 
 bool _isDartRuntime(LibraryElement l) =>
     l.isInSdk && l.source.uri.toString() == 'dart:_runtime';