Simplify type argument instantiation cache lookup by introducing an array

runtime/vm/snapshot.cc

Index: runtime/vm/snapshot.cc
===================================================================
--- runtime/vm/snapshot.cc	(revision 32709)
+++ runtime/vm/snapshot.cc	(working copy)
@@ -759,6 +759,9 @@
   if (object_id == kEmptyArrayObject) {
     return Object::empty_array().raw();
   }
+  if (object_id == kZeroArrayObject) {
+    return Object::zero_array().raw();
+  }
   if (object_id == kDynamicType) {
     return Object::dynamic_type();
   }
@@ -958,6 +961,12 @@
     return;
   }
 
+  // Check if it is a singleton zero array object.
+  if (rawobj == Object::zero_array().raw()) {
+    WriteVMIsolateObject(kZeroArrayObject);
+    return;
+  }
+
   // Check if it is a singleton dyanmic Type object.
   if (rawobj == Object::dynamic_type()) {
     WriteVMIsolateObject(kDynamicType);