Issue 29288. Resynthesize Import/Export/PartElement for every directive.

pkg/analyzer/test/src/summary/element_text.dart

Index: pkg/analyzer/test/src/summary/element_text.dart
diff --git a/pkg/analyzer/test/src/summary/element_text.dart b/pkg/analyzer/test/src/summary/element_text.dart
index 5f6ee00f650fab030d118c991bde88ef07665d48..ba31a09cd13506ad26bdd63c07926ea5f385e386 100644
--- a/pkg/analyzer/test/src/summary/element_text.dart
+++ b/pkg/analyzer/test/src/summary/element_text.dart
@@ -262,7 +262,7 @@ class _ElementWriter {
   void writeExportElement(ExportElement e) {
     writeMetadata(e, '', '\n');
     buffer.write('export ');
-    writeUri(e, e.exportedLibrary.source);
+    writeUri(e, e.exportedLibrary?.source);
 
     e.combinators.forEach(writeNamespaceCombinator);
 
@@ -463,7 +463,7 @@ class _ElementWriter {
     if (!e.isSynthetic) {
       writeMetadata(e, '', '\n');
       buffer.write('import ');
-      writeUri(e, e.importedLibrary.source);
+      writeUri(e, e.importedLibrary?.source);
 
       writeIf(e.isDeferred, ' deferred');
 
@@ -745,7 +745,7 @@ class _ElementWriter {
   void writeUnitElement(CompilationUnitElement e) {
     if (e.library.definingCompilationUnit != e) {
       buffer.writeln('-' * 20);
-      buffer.writeln('unit: ${e.source.shortName}');
+      buffer.writeln('unit: ${e.source?.shortName}');
       buffer.writeln();
     }
     e.functionTypeAliases.forEach(writeFunctionTypeAliasElement);