Don't crash on deserialized type inference.

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 '../dart_types.dart';
 import '../diagnostics/source_span.dart';
@@ skipping 347 matching lines @@
   String toString() => 'TreeElementMapping($analyzedElement)';
 
   @override
   void forEachConstantNode(f(Node n, ConstantExpression c)) {
     if (_constants != null) {
       _constants.forEach(f);
     }
   }
 
   @override
-  FunctionElement getFunctionDefinition(FunctionExpression node) {
+  Element getFunctionDefinition(FunctionExpression node) {
     return this[node];
   }
 
   @override
   ConstructorElement getRedirectingTargetConstructor(
       RedirectingFactoryBody node) {
     return this[node];
   }
 
   void defineTarget(Node node, JumpTarget target) {
@@ skipping 142 matching lines @@
       _nativeData = <Node, dynamic>{};
     }
     _nativeData[node] = nativeData;
   }
 
   @override
   dynamic getNativeData(Node node) {
     return _nativeData != null ? _nativeData[node] : null;
   }
 }