Add a script for running art standalone.

Change-Id: I0c48856d811c40662a6e4a4782478a0bc8e0f697
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 4e90478..86fae82 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -754,6 +754,12 @@
     parsed->image_ += GetAndroidRoot();
     parsed->image_ += "/framework/boot.art";
   }
+  if (!kIsTargetBuild && parsed->host_prefix_.empty()) {
+    const char* build_top = getenv("ANDROID_BUILD_TOP");
+    if (build_top != NULL) {
+      parsed->host_prefix_ = build_top;
+    }
+  }
   if (parsed->heap_growth_limit_ == 0) {
     parsed->heap_growth_limit_ = parsed->heap_maximum_size_;
   }
@@ -823,7 +829,7 @@
 bool Runtime::Start() {
   VLOG(startup) << "Runtime::Start entering";
 
-  CHECK(host_prefix_.empty()) << host_prefix_;
+  CHECK(!kIsTargetBuild || host_prefix_.empty()) << host_prefix_;
 
   // Restore main thread state to kNative as expected by native code.
   Thread* self = Thread::Current();