fix support for --package-root command line option to analyzer dartbug.com/4225

compiler/java/com/google/dart/compiler/CompilerConfiguration.java

Index: compiler/java/com/google/dart/compiler/CompilerConfiguration.java
===================================================================
--- compiler/java/com/google/dart/compiler/CompilerConfiguration.java	(revision 9979)
+++ compiler/java/com/google/dart/compiler/CompilerConfiguration.java	(working copy)
@@ -74,6 +74,11 @@
   LibrarySource getSystemLibraryFor(String importSpec);
 
   /**
+   * Return the system library manager 
+   */
+  public SystemLibraryManager getSystemLibraryManager();
+  
+  /**
    * Return {@link CompilerOptions} instance.
    * @return command line options passed to the compiler.
    */