[MASSEMBLY-858] use plugin classloader instead of thread classloader
fixes issues caused by MNG-6209 fix: but currently, every m-assembly-p from 2.2 to 3.0.0 using descriptor from classloader will fail with Maven 3.5.1...

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1798337 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/src/main/java/org/apache/maven/plugins/assembly/io/DefaultAssemblyReader.java b/src/main/java/org/apache/maven/plugins/assembly/io/DefaultAssemblyReader.java
index 9d4d11a..2dfd47a 100644
--- a/src/main/java/org/apache/maven/plugins/assembly/io/DefaultAssemblyReader.java
+++ b/src/main/java/org/apache/maven/plugins/assembly/io/DefaultAssemblyReader.java
@@ -195,8 +195,7 @@
                                                             throws AssemblyReadException,
                                                             InvalidAssemblerConfigurationException
     {
-        final InputStream resourceAsStream =
-            Thread.currentThread().getContextClassLoader().getResourceAsStream( "assemblies/" + ref + ".xml" );
+        final InputStream resourceAsStream = getClass().getResourceAsStream( "/assemblies/" + ref + ".xml" );
 
         if ( resourceAsStream == null )
         {