Fix background compilation issues when creating snapshots.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 22817d869bbd0abaf5d4fda1d02791227f2e72b7..03b275e17cdd9e3b739efc31fa17e5e2aa19db34 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -1524,12 +1524,17 @@ void Isolate::LowLevelShutdown() {
 }
 
 
-void Isolate::Shutdown() {
-  ASSERT(this == Isolate::Current());
+void Isolate::StopBackgroundCompiler() {
   // Wait until all background compilation has finished.
   if (background_compiler_ != NULL) {
     BackgroundCompiler::Stop(background_compiler_);
   }
+}
+
+
+void Isolate::Shutdown() {
+  ASSERT(this == Isolate::Current());
+  StopBackgroundCompiler();
 
 #if defined(DEBUG)
   if (heap_ != NULL) {