Synthetic function elements don't have a name or enclosing element/library.

pkg/analyzer/test/src/task/strong/front_end_inference_test.dart

Index: pkg/analyzer/test/src/task/strong/front_end_inference_test.dart
diff --git a/pkg/analyzer/test/src/task/strong/front_end_inference_test.dart b/pkg/analyzer/test/src/task/strong/front_end_inference_test.dart
index 2b30ed75748961fffb2935d3f315695fcea18b95..9e8301744a20eda82ef50556cf92de5436916fc2 100644
--- a/pkg/analyzer/test/src/task/strong/front_end_inference_test.dart
+++ b/pkg/analyzer/test/src/task/strong/front_end_inference_test.dart
@@ -119,16 +119,18 @@ class _InstrumentationValueForType extends fasta.InstrumentationValue {
   }
 
   void _appendElementName(StringBuffer buffer, Element element) {
-    String name = element.name ?? '';
-    if (element.library == null) {
-      // TODO(scheglov) This is wrong - every element must be in a library.
-      buffer.write('<unknown>::$name');
+    // Synthetic FunctionElement(s) don't have a name or enclosing library.
+    if (element.isSynthetic && element is FunctionElement) {
       return;
     }
-    String libraryName = element.library.name;
-    if (libraryName == '') {
-      throw new StateError('The element $name must be in a named library.');
+
+    LibraryElement library = element.library;
+    if (library == null) {
+      throw new StateError('Unexpected element without library: $element');
     }
+    String libraryName = library.name;
+
+    String name = element.name ?? '';
     if (libraryName != 'dart.core' &&
         libraryName != 'dart.async' &&
         libraryName != 'test') {