All strong mode cleaning of dart2js.

pkg/compiler/lib/src/resolution/tree_elements.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 dart2js.resolution.tree_elements;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../elements/resolution_types.dart';
 import '../diagnostics/source_span.dart';
@@ skipping 334 matching lines @@
   String toString() => 'TreeElementMapping($analyzedElement)';
 
   @override
   void forEachConstantNode(f(Node n, ConstantExpression c)) {
     if (_constants != null) {
       _constants.forEach(f);
     }
   }
 
   @override
-  Element getFunctionDefinition(FunctionExpression node) {
-    return this[node];
+  FunctionElement getFunctionDefinition(FunctionExpression node) {
+    Element e = this[node];
+    return e is FunctionElement ? e : null;
   }
 
   @override
   ConstructorElement getRedirectingTargetConstructor(
       RedirectingFactoryBody node) {
     return this[node];
   }
 
   void defineTarget(Node node, JumpTarget target) {
     _definedTargets ??= new Maplet<Node, JumpTarget>();
@@ skipping 62 matching lines @@
       _nativeData = <Node, dynamic>{};
     }
     _nativeData[node] = nativeData;
   }
 
   @override
   dynamic getNativeData(Node node) {
     return _nativeData != null ? _nativeData[node] : null;
   }
 }