Fixes #378

lib/src/codegen/js_codegen.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.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap, SplayTreeSet;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
 import 'package:analyzer/src/generated/ast.dart' hide ConstantEvaluator;
@@ skipping 758 matching lines @@
     for (FieldDeclaration member in fields) {
       for (VariableDeclaration fieldDecl in member.fields.variables) {
         var field = fieldDecl.element as FieldElement;
         if (_fieldsNeedingStorage.contains(field)) {
           body.add(_overrideField(field));
         }
       }
     }
 
     // Emit the signature on the class recording the runtime type information
+    var extensions = _extensionsToImplement(classElem);
     {
       var tStatics = <JS.Property>[];
       var tMethods = <JS.Property>[];
       var sNames = <JS.Expression>[];
       for (MethodDeclaration node in methods) {
         if (!(node.isSetter || node.isGetter || node.isAbstract)) {
           var name = node.name.name;
           var element = node.element;
           var inheritedElement =
               classElem.lookUpInheritedConcreteMethod(name, currentLibrary);
@@ skipping 31 matching lines @@
       var sigFields = <JS.Property>[];
       if (!tCtors.isEmpty) sigFields.add(build('constructors', tCtors));
       if (!tMethods.isEmpty) sigFields.add(build('methods', tMethods));
       if (!tStatics.isEmpty) {
         assert(!sNames.isEmpty);
         var aNames = new JS.Property(
             _propertyName('names'), new JS.ArrayInitializer(sNames));
         sigFields.add(build('statics', tStatics));
         sigFields.add(aNames);
       }
-      if (!sigFields.isEmpty) {
+      if (!sigFields.isEmpty || extensions.isNotEmpty) {
         var sig = new JS.ObjectInitializer(sigFields);
         var classExpr = new JS.Identifier(name);
         body.add(js.statement('dart.setSignature(#, #);', [classExpr, sig]));
       }
     }
 
     // If a concrete class implements one of our extensions, we might need to
     // add forwarders.
-    var extensions = _extensionsToImplement(classElem);
     if (extensions.isNotEmpty) {
       var methodNames = <JS.Expression>[];
       for (var e in extensions) {
         methodNames.add(_elementMemberName(e));
       }
       body.add(js.statement('dart.defineExtensionMembers(#, #);', [
         name,
         new JS.ArrayInitializer(methodNames, multiline: methodNames.length > 4)
       ]));
     }
@@ skipping 2531 matching lines @@
     t = fillDynamicTypeArgs(t, rules.provider) as InterfaceType;
     t.interfaces.forEach(_addExtensionType);
     t.mixins.forEach(_addExtensionType);
     _addExtensionType(t.superclass);
   }
 
   String generateLibrary(LibraryUnit unit) {
     // Clone the AST first, so we can mutate it.
     unit = unit.clone();
     var library = unit.library.element.library;
-    var fields = findFieldsNeedingStorage(unit);
+    var fields = findFieldsNeedingStorage(unit, _extensionTypes);
     var codegen =
         new JSCodegenVisitor(compiler, rules, library, _extensionTypes, fields);
     var module = codegen.emitLibrary(unit);
     var out = compiler.getOutputPath(library.source.uri);
     return writeJsLibrary(module, out,
         emitSourceMaps: options.emitSourceMaps,
         arrowFnBindThisWorkaround: options.arrowFnBindThisWorkaround);
   }
 }
 
@@ skipping 15 matching lines @@
 
 /// A special kind of element created by the compiler, signifying a temporary
 /// variable. These objects use instance equality, and should be shared
 /// everywhere in the tree where they are treated as the same variable.
 class TemporaryVariableElement extends LocalVariableElementImpl {
   TemporaryVariableElement.forNode(Identifier name) : super.forNode(name);
 
   int get hashCode => identityHashCode(this);
   bool operator ==(Object other) => identical(this, other);
 }