Fix and assert getLocalFunction invariant

pkg/compiler/lib/src/kernel/element_map_impl.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.kernel.element_map;
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../common.dart';
 import '../common/names.dart' show Identifiers;
@@ skipping 878 matching lines @@
     // TODO(johnniwinther): Cache the computed function type.
     int requiredParameters = node.requiredParameterCount;
     int positionalParameters = node.positionalParameters.length;
     List<String> namedParameters =
         node.namedParameters.map((p) => p.name).toList()..sort();
     return new ParameterStructure(
         requiredParameters, positionalParameters, namedParameters);
   }
 
   Local _getLocalFunction(ir.TreeNode node) {
+    assert(
+        node is ir.FunctionDeclaration || node is ir.FunctionExpression,
+        failedAt(
+            CURRENT_ELEMENT_SPANNABLE, 'Invalid local function node: $node'));
     return _localFunctionMap.putIfAbsent(node, () {
       MemberEntity memberContext;
       Entity executableContext;
       ir.TreeNode parent = node.parent;
       while (parent != null) {
         if (parent is ir.Member) {
           executableContext = memberContext = getMember(parent);
           break;
         }
         if (parent is ir.FunctionDeclaration ||
@@ skipping 1118 matching lines @@
       if (data.constructorBody != null) {
         f(data.constructorBody);
       }
     });
   }
 
   String getDeferredUri(ir.LibraryDependency node) {
     throw new UnimplementedError('JsKernelToElementMap.getDeferredUri');
   }
 }