Store parts in Kernel Library, resynthesize parts in Analyzer.

pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart

Index: pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
index eb7a89993d1e2e14cc9b39bf73c997a7d45ba42f..d8df4f86fc88338d1476241da82eef9b8281e558 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_library_builder.dart
@@ -759,6 +759,7 @@ class KernelLibraryBuilder
             combinators: toKernelCombinators(import.combinators)));
       }
     }
+
     for (Export export in exports) {
       var exportedBuilder = export.exported;
       Library exportedLibrary;
@@ -772,6 +773,14 @@ class KernelLibraryBuilder
             combinators: toKernelCombinators(export.combinators)));
       }
     }
+
+    for (var part in parts) {
+      // TODO(scheglov): Add support for annotations, see
+      // https://github.com/dart-lang/sdk/issues/30284.
+      String fileUri = part.fileUri.toString();
+      library.addPart(new LibraryPart(<Expression>[], fileUri));
+    }
+
     library.name = name;
     library.procedures.sort(compareProcedures);
     return library;