Allow DalvikRunner to run tests in core library packages.

This is a bit of a hack, but I think it's not unreasonable, given the purpose
of this tool (and the fact that we both favor writing our tests in the package
they're testing).

Bug: 2362187
diff --git a/libcore/tools/runner/java/dalvik/runner/Dx.java b/libcore/tools/runner/java/dalvik/runner/Dx.java
index 83f1265..d36a99d 100644
--- a/libcore/tools/runner/java/dalvik/runner/Dx.java
+++ b/libcore/tools/runner/java/dalvik/runner/Dx.java
@@ -22,8 +22,13 @@
 final class Dx {
 
     public void dex(String output, Classpath classpath) {
+        // We pass --core-library so that we can write tests in the same package they're testing,
+        // even when that's a core library package. If you're actually just using this tool to
+        // execute arbitrary code, this has the unfortunate side-effect of preventing "dx" from
+        // protecting you from yourself.
         new Command.Builder()
                 .args("dx")
+                .args("--core-library")
                 .args("--dex")
                 .args("--output=" + output)
                 .args(Strings.objectsToStrings(classpath.getElements()))