Reapply "- Use a hash table to canonicalize instances/arrays to avoid having to iterate over a line…

runtime/vm/precompiler.cc

Index: runtime/vm/precompiler.cc
diff --git a/runtime/vm/precompiler.cc b/runtime/vm/precompiler.cc
index 3819070471ff96a03d17fee6c70f9fb66b5e383d..6cf0736eb4bb07a18fcea4d6aa1a56be94df8340 100644
--- a/runtime/vm/precompiler.cc
+++ b/runtime/vm/precompiler.cc
@@ -1483,15 +1483,21 @@ void Precompiler::TraceTypesFromRetainedClasses() {
           retained_constants.Add(constant);
         }
       }
-      if (retained_constants.Length() > 0) {
+      intptr_t cid = cls.id();
+      if ((cid == kMintCid) || (cid == kBigintCid) || (cid == kDoubleCid)) {
+        // Constants stored as a plain list, no rehashing needed.
         constants = Array::MakeArray(retained_constants);
         cls.set_constants(constants);
       } else {
-        constants = Object::empty_array().raw();
+        // Rehash.
         cls.set_constants(Object::empty_array());
+        for (intptr_t j = 0; j < retained_constants.Length(); j++) {
+          constant ^= retained_constants.At(j);
+          cls.InsertCanonicalConstant(Z, constant);
+        }
       }
 
-      if (constants.Length() > 0) {
+      if (retained_constants.Length() > 0) {
         ASSERT(retain);  // This shouldn't be the reason we keep a class.
         retain = true;
       }