Only reload libraries when they may have been modified.

runtime/vm/isolate.cc

Index: runtime/vm/isolate.cc
diff --git a/runtime/vm/isolate.cc b/runtime/vm/isolate.cc
index 697aef86d87c8180253e0a3e2dff0e2ce41d9520..0c003fad07ade73a62c1eaa0795faf59e3ab84b7 100644
--- a/runtime/vm/isolate.cc
+++ b/runtime/vm/isolate.cc
@@ -846,7 +846,8 @@ Isolate::Isolate(const Dart_IsolateFlags& api_flags)
       has_attempted_reload_(false),
       no_reload_scope_depth_(0),
       reload_every_n_stack_overflow_checks_(FLAG_reload_every),
-      reload_context_(NULL) {
+      reload_context_(NULL),
+      reload_timestamp_(OS::GetCurrentTimeMillis()) {
   NOT_IN_PRODUCT(FlagsCopyFrom(api_flags));
   // TODO(asiva): A Thread is not available here, need to figure out
   // how the vm_tag (kEmbedderTagId) can be set, these tags need to
@@ -1089,12 +1090,13 @@ void Isolate::ReportReloadError(const Error& error) {
 }
 
 
-void Isolate::ReloadSources(bool dont_delete_reload_context) {
+void Isolate::ReloadSources(bool force_reload,
+                            bool dont_delete_reload_context) {
   // TODO(asiva): Add verification of canonical objects.
   ASSERT(!IsReloading());
   has_attempted_reload_ = true;
   reload_context_ = new IsolateReloadContext(this);
-  reload_context_->StartReload();
+  reload_context_->StartReload(force_reload);
   // TODO(asiva): Add verification of canonical objects.
   if (dont_delete_reload_context) {
     // Unit tests use the reload context later. Caller is responsible
@@ -1121,6 +1123,7 @@ void Isolate::DoneFinalizing() {
         sticky_reload_error_ = reload_context_->error();
       } else {
         reload_context_->ReportSuccess();
+        reload_timestamp_ = reload_context_->reload_timestamp();
       }
     }
   )