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

compiler/java/com/google/dart/compiler/testing/TestCompilerConfiguration.java

Index: compiler/java/com/google/dart/compiler/testing/TestCompilerConfiguration.java
===================================================================
--- compiler/java/com/google/dart/compiler/testing/TestCompilerConfiguration.java	(revision 9979)
+++ compiler/java/com/google/dart/compiler/testing/TestCompilerConfiguration.java	(working copy)
@@ -91,4 +91,9 @@
   public ErrorFormat printErrorFormat() {
     return ErrorFormat.MACHINE;
   }
+
+  @Override
+  public SystemLibraryManager getSystemLibraryManager() {
+    return systemLibraryManager;
+  }
 }