Support type alias syntax

"Support" in the sense of dropping these on the floor and compiling, rather than 
bailing out with a todo. 

We already don't make any guarantees about which type annotations we'll preserve 
through to the output, so it seems fine for now to just drop type aliases.

compiler/packages/babel-plugin-react-forget/src/HIR/BuildHIR.ts

@@ -227,7 +227,7 @@ function lowerStatement(
   builder: HIRBuilder,
   stmtPath: NodePath<t.Statement>,
   label: string | null = null
-): undefined {
+): void {
   const stmtNode = stmtPath.node;
   switch (stmtNode.type) {
     case "ThrowStatement": {
@@ -1285,6 +1285,11 @@ function lowerStatement(
 
       return;
     }
+    case "TypeAlias":
+    case "TSTypeAliasDeclaration": {
+      // We do not preserve type annotations/syntax through transformation
+      return;
+    }
     case "ClassDeclaration":
     case "DeclareClass":
     case "DeclareExportAllDeclaration":
@@ -1303,7 +1308,6 @@ function lowerStatement(
     case "ImportDeclaration":
     case "InterfaceDeclaration":
     case "OpaqueType":
-    case "TypeAlias":
     case "TSDeclareFunction":
     case "TSEnumDeclaration":
     case "TSExportAssignment":
@@ -1311,7 +1315,6 @@ function lowerStatement(
     case "TSInterfaceDeclaration":
     case "TSModuleDeclaration":
     case "TSNamespaceExportDeclaration":
-    case "TSTypeAliasDeclaration":
     case "WithStatement": {
       builder.errors.push({
         reason: `(BuildHIR::lowerStatement) Handle ${stmtPath.type} statements`,

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/type-alias-declaration.expect.md

@@ -0,0 +1,44 @@
+
+## Input
+
+```javascript
+function Component(props) {
+  type User = { name: string };
+  const user: User = { name: props.name };
+  return user;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ name: "Mofei" }],
+};
+
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react";
+function Component(props) {
+  const $ = useMemoCache(2);
+  let t0;
+  if ($[0] !== props.name) {
+    t0 = { name: props.name };
+    $[0] = props.name;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const user = t0;
+  return user;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ name: "Mofei" }],
+};
+
+```
+      
+### Eval output
+(kind: ok) {"name":"Mofei"}

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/type-alias-declaration.ts

@@ -0,0 +1,10 @@
+function Component(props) {
+  type User = { name: string };
+  const user: User = { name: props.name };
+  return user;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ name: "Mofei" }],
+};

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/type-alias.flow.expect.md

@@ -0,0 +1,44 @@
+
+## Input
+
+```javascript
+// @flow
+function Component(props) {
+  type User = {name: string};
+  const user: User = {name: props.name};
+  return user;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{name: 'Mofei'}],
+};
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react";
+function Component(props) {
+  const $ = useMemoCache(2);
+  let t0;
+  if ($[0] !== props.name) {
+    t0 = { name: props.name };
+    $[0] = props.name;
+    $[1] = t0;
+  } else {
+    t0 = $[1];
+  }
+  const user = t0;
+  return user;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{ name: "Mofei" }],
+};
+
+```
+      
+### Eval output
+(kind: ok) {"name":"Mofei"}

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/type-alias.flow.js

@@ -0,0 +1,11 @@
+// @flow
+function Component(props) {
+  type User = {name: string};
+  const user: User = {name: props.name};
+  return user;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [{name: 'Mofei'}],
+};