let/const

Summary: This commit allows for the creation of let/const bindings and ensures
all block-scoped bindings are installed into the appropriate scope. This
involves creating new LexScopes, as outlined in `test/bindings/scope.js`.

Before this change, the environment stack, i.e., frames, changesets, and
scopes, would change in lockstep. Now, lex scopes change separately from
frames and changesets via `push_lex` and `pop_lex`.

When adding heap refinements, we may traverse over lex scopes to find
the scope where the base object was bound.

In unusual cases, like try/catch, envs may have an uneven number of
scopes when merging. The uneven scopes will always be lex. I have
updated `merge_env` to accept this imbalance.

Note that this is not a complete implementation of let/const:
- [x] switch statements should forbid rebinding lexicals between cases
- [x] functions, amazingly, are block scoped *and* hoisted in ES2015
- [ ] numerous TDZ cases, surely, which I have not yet investigated
- [x] can't update cons
Closes #784

Reviewed By: @jeffmo

Differential Revision: D2428230

Pulled By: @bhosmer

src/typing/constraint_js.ml

@@ -745,11 +745,15 @@ module Scope = struct
 
     and implicit_let_kinds =
       | ClassNameBinding
+      | CatchParamBinding
+      | FunctionBinding
 
     let string_of_value_kind = function
     | Const -> "const"
     | Let None -> "let"
     | Let (Some ClassNameBinding) -> "class"
+    | Let (Some CatchParamBinding) -> "catch"
+    | Let (Some FunctionBinding) -> "function"
     | Var -> "var"
 
     type value_binding = {
@@ -828,6 +832,13 @@ module Scope = struct
         else Value { v with specific = make_specific v.general }
       | Type _ -> entry
 
+    let is_lex = function
+      | Type _ -> false
+      | Value v ->
+        match v.kind with
+        | Const -> true
+        | Let _ -> true
+        | _ -> false
   end
 
   (* keys for refinements *)
@@ -963,6 +974,10 @@ module Scope = struct
     | Some f -> scope |> update_entries (Entry.havoc ~name f)
     | None -> ()
 
+  let is_lex scope =
+    match scope.kind with
+    | LexScope -> true
+    | _ -> false
 end
 
 (***************************************)

src/typing/constraint_js.mli

@@ -288,7 +288,10 @@ module Scope: sig
     val string_of_state: state -> string
 
     type value_kind = Const | Let of implicit_let_kinds option | Var
-    and implicit_let_kinds = ClassNameBinding
+    and implicit_let_kinds =
+      | ClassNameBinding
+      | CatchParamBinding
+      | FunctionBinding
 
     val string_of_value_kind: value_kind -> string
 
@@ -326,6 +329,8 @@ module Scope: sig
 
     val string_of_kind: t -> string
     val havoc: ?name:string -> (Type.t -> Type.t) -> string -> t -> t
+
+    val is_lex: t -> bool
   end
 
   module Key: sig
@@ -375,6 +380,7 @@ module Scope: sig
 
   val havoc: ?name: string -> ?make_specific: (Type.t -> Type.t) -> t -> unit
 
+  val is_lex: t -> bool
 end
 
 (***************************************)