Canonicalize mixin type during class finalization. Add asserts that most types are canonical.

runtime/vm/class_finalizer.cc

Index: runtime/vm/class_finalizer.cc
diff --git a/runtime/vm/class_finalizer.cc b/runtime/vm/class_finalizer.cc
index 4d508c80a8f68e5ceccecd8196e2f5825ee89adc..5fecf1cb5292fe9ad1e165fb44e095d8c25dd5c4 100644
--- a/runtime/vm/class_finalizer.cc
+++ b/runtime/vm/class_finalizer.cc
@@ -2236,6 +2236,12 @@ void ClassFinalizer::FinalizeTypesInClass(const Class& cls) {
     super_type = FinalizeType(cls, super_type, kCanonicalizeWellFormed);
     cls.set_super_type(super_type);
   }
+  // Finalize mixin type.
+  Type& mixin_type = Type::Handle(cls.mixin());
+  if (!mixin_type.IsNull()) {
+    mixin_type ^= FinalizeType(cls, mixin_type, kCanonicalizeWellFormed);
+    cls.set_mixin(mixin_type);
+  }
   if (cls.IsSignatureClass()) {
     // Check for illegal self references.
     GrowableArray<intptr_t> visited_aliases;