[be] Clean up null -> PolyType in ObjectShape

--- 

Question - should the unifier ignore `PolyType` for now? 

```js 

class Unifier { 

// ... 

bindVariableTo(v: TypeVar, type: Type) { 

if (type.kind === "Poly") { 

return; 

} 

// ... 

```

compiler/forget/src/HIR/Environment.ts

@@ -8,6 +8,7 @@ import {
   IdentifierId,
   makeIdentifierId,
   ObjectType,
+  PolyType,
   ValueKind,
 } from "./HIR";
 import { Hook } from "./Hooks";
@@ -84,7 +85,7 @@ export class Environment {
   getPropertyType(
     receiver: ObjectType | FunctionType,
     property: string
-  ): BuiltInType | null {
+  ): BuiltInType | PolyType | null {
     const { shapeId } = receiver;
     if (shapeId !== null) {
       // If an object or function has a shapeId, it must have been assigned

compiler/forget/src/HIR/ObjectShape.ts

@@ -38,7 +38,7 @@ function createAnonId(): string {
  */
 export function addFunction(
   registry: ShapeRegistry,
-  properties: Iterable<[string, BuiltInType | null]>,
+  properties: Iterable<[string, BuiltInType | PolyType]>,
   fn: FunctionSignature
 ): FunctionType {
   const shapeId = createAnonId();
@@ -58,7 +58,7 @@ export function addFunction(
 export function addObject(
   registry: ShapeRegistry,
   id: string | null,
-  properties: Iterable<[string, BuiltInType | null]>
+  properties: Iterable<[string, BuiltInType | PolyType]>
 ): ObjectType {
   const shapeId = id ?? createAnonId();
   addShape(registry, shapeId, properties, null);
@@ -71,7 +71,7 @@ export function addObject(
 function addShape(
   registry: ShapeRegistry,
   id: string,
-  properties: Iterable<[string, BuiltInType | null]>,
+  properties: Iterable<[string, BuiltInType | PolyType]>,
   functionType: FunctionSignature | null
 ): ObjectShape {
   const shape: ObjectShape = {
@@ -110,7 +110,7 @@ export type FunctionSignature = {
  * are both represented by {@link ObjectShape.functionType}.
  */
 export type ObjectShape = {
-  properties: Map<string, BuiltInType | null>;
+  properties: Map<string, BuiltInType | PolyType>;
   functionType: FunctionSignature | null;
 };
 

compiler/forget/src/TypeInference/InferTypes.ts

@@ -236,6 +236,11 @@ class Unifier {
   }
 
   bindVariableTo(v: TypeVar, type: Type): void {
+    if (type.kind === "Poly") {
+      //  Ignore PolyType, since we don't support polymorphic types correctly.
+      return;
+    }
+
     if (this.substitutions.has(v.id)) {
       this.unify(this.substitutions.get(v.id)!, type);
       return;