Fix PackageLibraryManager set package directory

compiler/java/com/google/dart/compiler/PackageLibraryManager.java

 // Copyright (c) 2012, 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.
 
 package com.google.dart.compiler;
 
 import java.io.File;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.util.ArrayList;
@@ skipping 205 matching lines @@
   }
   
   
   public List<File> getPackageRoots(){
     return packageRoots;
   }
   
   
   public void setPackageRoots(List<File> roots){
     if (roots == null || roots.isEmpty()){
-      this.packageRoots = DEFAULT_PACKAGE_ROOTS;
-    } else { 
-      packageRoots.clear();
-      for (File file : roots){   
-        packageRoots.add(file.getAbsoluteFile());
-      }   
+      roots = DEFAULT_PACKAGE_ROOTS;
     }
+    packageRoots.clear();
+    for (File file : roots){   
+      packageRoots.add(file.getAbsoluteFile());
+    }   
     packageRootsUri.clear();
     for (File file : roots){
       packageRootsUri.add(file.toURI());
     }
   }
   
   /**
    * Translate the URI from dart://[host]/[pathToLib] (e.g. dart://html/html.dart)
    * to a "file:" URI (e.g. "file:/some/install/directory/html.dart")
    *
@@ skipping 70 matching lines @@
     return SDK_LIBRARY_MANAGER.getAllSystemLibraries();
   }
   
   public File getSdkLibPath() {
     return SDK_LIBRARY_MANAGER.getSdkLibPath();
   }
   
   
   
 }