Fix referencing prefixed types from parts.

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

Index: pkg/analyzer/lib/src/summary/summarize_elements.dart
diff --git a/pkg/analyzer/lib/src/summary/summarize_elements.dart b/pkg/analyzer/lib/src/summary/summarize_elements.dart
index b4b858cc96d87e7c8c1294ff7e53a76183b77ed8..80f846e6382d6df059ca5ef8c1768907c4f43d8f 100644
--- a/pkg/analyzer/lib/src/summary/summarize_elements.dart
+++ b/pkg/analyzer/lib/src/summary/summarize_elements.dart
@@ -674,7 +674,6 @@ class _LibrarySerializer {
           if (element is TypeParameterizedElement) {
             numTypeParameters = element.typeParameters.length;
           }
-          int index = unlinkedReferences.length;
           // Figure out a prefix that may be used to refer to the given type.
           // TODO(paulberry): to avoid subtle relinking inconsistencies we
           // should use the actual prefix from the AST (a given type may be
@@ -685,6 +684,7 @@ class _LibrarySerializer {
           if (prefix != null) {
             prefixReference = serializePrefix(prefix);
           }
+          int index = unlinkedReferences.length;
           unlinkedReferences.add(encodeUnlinkedReference(
               name: element.name, prefixReference: prefixReference));
           prelinkedReferences.add(encodePrelinkedReference(