Only reload libraries when they may have been modified.

runtime/vm/isolate.h

Index: runtime/vm/isolate.h
diff --git a/runtime/vm/isolate.h b/runtime/vm/isolate.h
index b49d59b130bf265f9f316dee4d2d03b39ab94e08..3186cf1cddbe8ac9bb88aa1b165ba48f79bb6bf4 100644
--- a/runtime/vm/isolate.h
+++ b/runtime/vm/isolate.h
@@ -257,7 +257,8 @@ class Isolate : public BaseIsolate {
 
   // By default the reload context is deleted. This parameter allows
   // the caller to delete is separately if it is still needed.
-  void ReloadSources(bool dont_delete_reload_context = false);
+  void ReloadSources(bool force_reload,
+                     bool dont_delete_reload_context = false);
 
   bool MakeRunnable();
   void Run();
@@ -492,6 +493,13 @@ class Isolate : public BaseIsolate {
 
   void ReportReloadError(const Error& error);
 
+  void set_last_reload_timestamp(int64_t value) {
+    last_reload_timestamp_ = value;
+  }
+  int64_t last_reload_timestamp() const {
+    return last_reload_timestamp_;
+  }
+
   uword user_tag() const {
     return user_tag_;
   }
@@ -831,6 +839,7 @@ class Isolate : public BaseIsolate {
   // Per-isolate copy of FLAG_reload_every.
   intptr_t reload_every_n_stack_overflow_checks_;
   IsolateReloadContext* reload_context_;
+  int64_t last_reload_timestamp_;
 
 #define ISOLATE_METRIC_VARIABLE(type, variable, name, unit)                    \
   type metric_##variable##_;