Add logic to exclude certain libraries from API doc.

pkg/dartdoc/lib/dartdoc.dart

Index: pkg/dartdoc/lib/dartdoc.dart
diff --git a/pkg/dartdoc/lib/dartdoc.dart b/pkg/dartdoc/lib/dartdoc.dart
index 13d5d0acf804aac1f8baef2a38db63a391a6465b..0fb159ef9b2464a879f8332df855bf2cebbec50b 100644
--- a/pkg/dartdoc/lib/dartdoc.dart
+++ b/pkg/dartdoc/lib/dartdoc.dart
@@ -267,15 +267,15 @@ class Dartdoc {
     }
     var includeByDefault = true;
     String libraryName = displayName(library);
+    if (excludedLibraries.contains(libraryName)) {
+      return false;
+    }
     if (!includedLibraries.isEmpty) {
       includeByDefault = false;
-      if (includedLibraries.indexOf(libraryName) != -1) {
+      if (includedLibraries.contains(libraryName)) {
         return true;
       }
     }
-    if (excludedLibraries.indexOf(libraryName) != -1) {
-      return false;
-    }
     if (libraryName.startsWith('dart:')) {
       String suffix = libraryName.substring('dart:'.length);
       LibraryInfo info = LIBRARIES[suffix];