Modify summary Builder classes to avoid the use of Object.

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 f7d7a94c21495a81389f1accc229b4398eb1e288..0854a2476058c36b1d717861d2e945af750b4de9 100644
--- a/pkg/analyzer/test/src/summary/summary_test.dart
+++ b/pkg/analyzer/test/src/summary/summary_test.dart
@@ -46,9 +46,9 @@ class SummarizeElementsTest extends ResolverTestCase with SummaryTest {
    */
   void serializeLibraryElement(LibraryElement library) {
     BuilderContext builderContext = new BuilderContext();
-    Object serializedLib = summarize_elements.serializeLibrary(
+    PrelinkedLibraryBuilder serializedLib = summarize_elements.serializeLibrary(
         builderContext, library, typeProvider);
-    List<int> encodedLib = builderContext.getBuffer(serializedLib);
+    List<int> encodedLib = serializedLib.toBuffer();
     lib = new PrelinkedLibrary.fromBuffer(encodedLib);
   }