Fix summary generation in "package-summary-only" mode.

If we aren't doing full analysis, then we can't use
`context.librarySources` to find out the set of library sources (because
it doesn't include sources that haven't been examined yet).  Instead, we
have to query each source to find out its type.

R=scheglov@google.com

Review URL: https://codereview.chromium.org/1770323003 .

pkg/analyzer_cli/lib/src/package_analyzer.dart

@@ -94,12 +94,13 @@ class PackageAnalyzer {
     // Write summary for Dart libraries.
     if (options.packageSummaryOutput != null) {
       PackageBundleAssembler assembler = new PackageBundleAssembler();
-      for (Source source in context.librarySources) {
+      for (Source source in explicitSources) {
+        if (context.computeKindOf(source) != SourceKind.LIBRARY) {
+          continue;
+        }
         if (pathos.isWithin(packageLibPath, source.fullName)) {
           LibraryElement libraryElement = context.computeLibraryElement(source);
-          if (libraryElement != null) {
-            assembler.serializeLibraryElement(libraryElement);
-          }
+          assembler.serializeLibraryElement(libraryElement);
         }
       }
       // Write the whole package bundle.