Fix a couple of isolate-related memory leaks that show up a lot in valgrind output.

runtime/lib/isolate.cc

Index: runtime/lib/isolate.cc
diff --git a/runtime/lib/isolate.cc b/runtime/lib/isolate.cc
index cbc8f00070cc66dfb28d457afb6b0960f628ef7c..75a9a34c3622a3671c7fcc314fd4d9e513482b8e 100644
--- a/runtime/lib/isolate.cc
+++ b/runtime/lib/isolate.cc
@@ -196,6 +196,7 @@ static void RunIsolate(uword parameter) {
     }
     ASSERT(result.IsNull());
     free(class_name);
+    free(library_url);
     result = isolate->StandardRunLoop();
     if (result.IsError()) {
       ProcessError(result);