Split _LibrarySerializer into two classes.

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

Index: pkg/analyzer/test/src/summary/summary_test.dart
diff --git a/pkg/analyzer/test/src/summary/summary_test.dart b/pkg/analyzer/test/src/summary/summary_test.dart
index 41692ab1fd7fbd628539443b956b49c0bea51251..3d8d0623887c8fa40749346cf861949ee905e94d 100644
--- a/pkg/analyzer/test/src/summary/summary_test.dart
+++ b/pkg/analyzer/test/src/summary/summary_test.dart
@@ -582,11 +582,15 @@ abstract class SummaryTest {
    * type.
    */
   void checkUnresolvedTypeRef(
-      TypeRef typeRef, String expectedPrefix, String expectedName) {
+      TypeRef typeRef, String expectedPrefix, String expectedName,
+      {LinkedUnit linkedSourceUnit, UnlinkedUnit unlinkedSourceUnit}) {
     // When serializing from the element model, unresolved type refs lose their
     // name.
     checkTypeRef(typeRef, null, null, checkAstDerivedData ? expectedName : null,
-        expectedPrefix: expectedPrefix, expectedKind: ReferenceKind.unresolved);
+        expectedPrefix: expectedPrefix,
+        expectedKind: ReferenceKind.unresolved,
+        linkedSourceUnit: linkedSourceUnit,
+        unlinkedSourceUnit: unlinkedSourceUnit);
   }
 
   fail_constExpr_pushInt_shiftOr() {
@@ -3909,6 +3913,21 @@ typedef F();''';
     expect(type.typeParameters[1].name, 'U');
   }
 
+  test_unresolved_reference_in_multiple_parts() {
+    addNamedSource('/a.dart', 'part of foo; int x; Unresolved y;');
+    serializeLibraryText('library foo; part "a.dart"; Unresolved z;',
+        allowErrors: true);
+    // The unresolved types in the defining compilation unit and the part
+    // should both work correctly even though they use different reference
+    // indices.
+    checkUnresolvedTypeRef(
+        unlinkedUnits[0].variables[0].type, null, 'Unresolved');
+    checkUnresolvedTypeRef(
+        unlinkedUnits[1].variables[1].type, null, 'Unresolved',
+        linkedSourceUnit: linked.units[1],
+        unlinkedSourceUnit: unlinkedUnits[1]);
+  }
+
   test_variable() {
     String text = 'int i;';
     UnlinkedVariable v = serializeVariableText(text, variableName: 'i');