[Bugbash] Mixed refines to read-only object under typeof = object

Summary:
It is not safe to refine `mixed` to `{[string] : mixed}` because this allows writes of any kind into the refined object, and we don't know what type of values the object holds. The only safe operation is to read `mixed` out of the object, so the properties must be read only.

```
function bad (x : mixed) {
  if (typeof x === "object" && x !== null) {
    x.a = 3;
  }
}

let obj : {a : string} = {a : "oops"};
bad(obj); // yikes
```

Reviewed By: mvitousek

Differential Revision: D14799387

fbshipit-source-id: 210ef1d98733b085b202dcbd4e1e83d69e707f08

src/typing/type_filter.ml

@@ -331,7 +331,7 @@ let object_ cx t =
       key = StrT.why r |> with_trust bogus_trust;
       value = DefT (replace_reason_const MixedT.desc r, bogus_trust (), MixedT Mixed_everything);
       dict_name = None;
-      dict_polarity = Neutral;
+      dict_polarity = Positive;
     } in
     let proto = ObjProtoT reason in
     let obj = Obj_type.mk_with_proto cx reason ?dict proto in

tests/refinements/mixed.js

@@ -92,7 +92,7 @@ function false_(x: mixed) {
 
 function obj2(x: mixed) {
   if (typeof x === "object") {
-    (x: { [key: string]: mixed } | null);
+    (x: { +[key: string]: mixed } | null);
     if (x !== null) {
       (x['foo']: string); // error, mixed
     }

tests/refinements/mixed_object.js

@@ -0,0 +1,9 @@
+// @flow
+function bad (x : mixed) {
+  if (typeof x === "object" && x !== null) {
+    x.a = 3;
+  }
+}
+
+let obj : {a : string} = {a : "oops"};
+bad(obj); // yikes

tests/refinements/refinements.exp

@@ -1238,6 +1238,14 @@ References:
                                 ^^^^^^ [2]
 
 
+Error ---------------------------------------------------------------------------------------------- mixed_object.js:4:5
+
+Cannot assign `3` to `x.a` because property `a` is not writable.
+
+   4|     x.a = 3;
+          ^^^
+
+
 Error ------------------------------------------------------------------------------------------------------- not.js:5:5
 
 Cannot perform arithmetic operation because null or undefined [1] is not a number.
@@ -3430,4 +3438,4 @@ Cannot perform arithmetic operation because undefined [1] is not a number.
 
 
 
-Found 248 errors
+Found 249 errors