Check PackageMapUriResolver constructor arguments.

pkg/analyzer/lib/source/package_map_resolver.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library source.package_map_resolver;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/src/generated/source.dart';
+import 'package:analyzer/src/util/asserts.dart' as asserts;
 
 
 /**
  * A [UriResolver] implementation for the `package:` scheme that uses a map of
  * package names to their directories.
  */
 class PackageMapUriResolver extends UriResolver {
   /**
    * The name of the `package` scheme.
    */
   static const String PACKAGE_SCHEME = "package";
 
   /**
    * A table mapping package names to the path of the directories containing
    * the package.
    */
   final Map<String, List<Folder>> packageMap;
 
   /**
    * The [ResourceProvider] for this resolver.
    */
   final ResourceProvider resourceProvider;
 
   /**
    * Create a new [PackageMapUriResolver].
    *
    * [packageMap] is a table mapping package names to the paths of the
    * directories containing the package
    */
-  PackageMapUriResolver(this.resourceProvider, this.packageMap);
+  PackageMapUriResolver(this.resourceProvider, this.packageMap) {
+    asserts.notNull(resourceProvider);
+    asserts.notNull(packageMap);
+  }
 
   @override
   Source resolveAbsolute(Uri uri) {
     if (!isPackageUri(uri)) {
       return null;
     }
     // Prepare path.
     String path = uri.path;
     // Prepare path components.
     String pkgName;
@@ skipping 40 matching lines @@
     return null;
   }
 
   /**
    * Returns `true` if [uri] is a `package` URI.
    */
   static bool isPackageUri(Uri uri) {
     return uri.scheme == PACKAGE_SCHEME;
   }
 }