Abnormals should pop lex scope in catch clause

This resolves an issue which I misdiagnosed, causing me to add
unnecessary cases to Env_js.merge_env, which could mask other real
errors.

What's happening here: We enter a catch clause, which pushes a new lex
scope. Before we can pop the lex scope, we hit an abnormal and jump
away. Then, we merge the env that has a dangling lex scope on top.

In general, we run this risk whenever we push a lex scope and then
process statements that might raise. I have convinced myself that no
other code that calls merge_env will hit this specific case, but a
generalized approach would be nice, too.

src/typing/env_js.ml

@@ -765,11 +765,6 @@ let merge_env =
         ()
       | None, Some e, None when is_lex scope1 && Entry.is_lex e ->
         ()
-      (* walk through uneven lex scopes *)
-      | Some _, Some _, None when is_lex scope2 ->
-        merge_entry cx reason (env0, env1, env2_) name
-      | Some _, None, Some _ when is_lex scope1 ->
-        merge_entry cx reason (env0, env1_, env2) name
       (* otherwise, non-refinement uneven distributions are asserts. *)
       | orig, child1, child2 ->
         let print_entry_kind_opt = function

src/typing/type_inference_js.ml

@@ -1350,8 +1350,13 @@ and statement cx type_params_map = Ast.Statement.(
           Env_js.push_lex ();
           Scope.Entry.(Env_js.bind_implicit_let
             ~state:Initialized CatchParamBinding cx name t r);
-          List.iter (statement_decl cx type_params_map) b.Block.body;
-          toplevels cx type_params_map b.Block.body;
+          Abnormal.exception_handler
+            (fun () ->
+              List.iter (statement_decl cx type_params_map) b.Block.body;
+              toplevels cx type_params_map b.Block.body)
+            (fun exn ->
+              Env_js.pop_lex ();
+              Abnormal.raise_exn exn);
           Env_js.pop_lex ()
 
       | loc, Identifier (_, { Ast.Identifier.name; _ }) ->