Precompile invoke-field-dispatchers for closures.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 2366142b7a56dacf9e74fbdcb5ace7e723b849d9..c65d4846464ea0e9034cc3fde71ccd7ffabee24e 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -2602,7 +2602,8 @@ void Class::CalculateFieldOffsets() const {
 
 RawFunction* Class::GetInvocationDispatcher(const String& target_name,
                                             const Array& args_desc,
-                                            RawFunction::Kind kind) const {
+                                            RawFunction::Kind kind,
+                                            bool create_if_absent) const {
   enum {
     kNameIndex = 0,
     kArgsDescIndex,
@@ -2632,7 +2633,7 @@ RawFunction* Class::GetInvocationDispatcher(const String& target_name,
     }
   }
 
-  if (dispatcher.IsNull()) {
+  if (dispatcher.IsNull() && create_if_absent) {
     if (i == cache.Length()) {
       // Allocate new larger cache.
       intptr_t new_len = (cache.Length() == 0)