Do not try to patch type objects that are already canonical.

runtime/vm/object.cc

Index: runtime/vm/object.cc
diff --git a/runtime/vm/object.cc b/runtime/vm/object.cc
index 61c605fa1a033f260981345f1d0dcd0d41b8fef5..7357dbdc2578da490e0d4d8a55a56c371f0dd6ec 100644
--- a/runtime/vm/object.cc
+++ b/runtime/vm/object.cc
@@ -15606,6 +15606,7 @@ RawAbstractType* Type::Canonicalize(GrowableObjectArray* trail) const {
       return this->raw();
     }
     ASSERT(this->Equals(type));
+    ASSERT(type.IsCanonical());
     return type.raw();
   }
 
@@ -15627,6 +15628,7 @@ RawAbstractType* Type::Canonicalize(GrowableObjectArray* trail) const {
     }
     ASSERT(type.IsFinalized());
     if (this->Equals(type)) {
+      ASSERT(type.IsCanonical());
       return type.raw();
     }
     index++;
@@ -15655,6 +15657,7 @@ RawAbstractType* Type::Canonicalize(GrowableObjectArray* trail) const {
     }
     ASSERT(type.IsFinalized());
     if (this->Equals(type)) {
+      ASSERT(type.IsCanonical());
       return type.raw();
     }
     index++;