Finish strong mode cleaning of dart2js.

pkg/compiler/lib/src/js_model/elements.dart

 // Copyright (c) 2017, 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 dart2js.js_model.elements;
 
 import '../common_elements.dart';
 import '../constants/constant_system.dart';
 import '../elements/elements.dart';
 import '../elements/entities.dart';
@@ skipping 252 matching lines @@
         .isFunctionUsedByBackend(_map.toFrontendMember(element));
   }
 
   @override
   bool isFieldUsedByBackend(FieldEntity element) {
     return _backendUsage.isFieldUsedByBackend(_map.toFrontendMember(element));
   }
 
   @override
   Iterable<FunctionEntity> get globalFunctionDependencies {
-    return _backendUsage.globalFunctionDependencies.map(_map.toBackendMember);
+    return _backendUsage.globalFunctionDependencies
+        .map((FunctionEntity e) => _map.toBackendMember(e));
   }
 
   @override
   Iterable<ClassEntity> get globalClassDependencies {
     return _backendUsage.globalClassDependencies.map(_map.toBackendClass);
   }
 
   @override
   bool get requiresPreamble => _backendUsage.requiresPreamble;
 
@@ skipping 979 matching lines @@
   @override
   ClassEntity get numClass => _map.toBackendClass(_commonElements.numClass);
 
   @override
   ClassEntity get boolClass => _map.toBackendClass(_commonElements.boolClass);
 
   @override
   FunctionEntity get throwUnsupportedError =>
       _map.toBackendMember(_commonElements.throwUnsupportedError);
 }