Fix linker's handling of malformed function references.

pkg/analyzer/lib/src/summary/link.dart

 // Copyright (c) 2016, 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.
 
 /**
  * This library is capable of producing linked summaries from unlinked
  * ones (or prelinked ones).  It functions by building a miniature
  * element model to represent the contents of the summaries, and then
  * scanning the element model to gather linked information and adding
  * it to the summary data structures.
@@ skipping 984 matching lines @@
           : unlinkedReference.name;
       int containingReference = unlinkedReference == null
           ? linkedReference.containingReference
           : unlinkedReference.prefixReference;
       if (containingReference != 0 &&
           _linkedUnit.references[containingReference].kind !=
               ReferenceKind.prefix) {
         if (linkedReference.kind == ReferenceKind.function) {
           // Local function
           _references[index] = _resolveRef(containingReference)
-              .getLocalFunction(linkedReference.localIndex);
+                  .getLocalFunction(linkedReference.localIndex) ??
+              UndefinedElementForLink.instance;
         } else {
           _references[index] =
               _resolveRef(containingReference).getContainedName(name);
         }
       } else if (linkedReference.dependency == 0) {
         if (name == 'void') {
           _references[index] = enclosingElement._linker.voidElement;
         } else if (name == '*bottom*') {
           _references[index] = enclosingElement._linker.bottomElement;
         } else if (name == 'dynamic') {
@@ skipping 3619 matching lines @@
    * there are no type parameters in scope.
    */
   TypeParameterizedElementMixin get _typeParameterContext;
 
   @override
   noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
 
   @override
   String toString() => '$enclosingElement.$name';
 }