make Chunk types opaque where possible

Previously Chunks were not opaque so you could construct conincidental types that satisifed the type condition but were not routed through a Chunk factory. This is dangerous because it allows for things like the transpoition of escaping and chunking which has led to bugs in certain environments.

Ideally all chunk types would be opaque however the ReactNative Fizz runtime constructs type-compatible entries with PrecomputedChunk without calling into the Chunk factory. This should be fixed but is outside the scope of this change so I left those not opaque where they would otherwise lead to new type errors

Author: gnoff

packages/react-dom-bindings/src/server/ReactDOMLegacyServerStreamConfig.js

@@ -12,8 +12,8 @@ export interface Destination {
   destroy(error: Error): mixed;
 }
 
-export type PrecomputedChunk = string;
-export type Chunk = string;
+export opaque type PrecomputedChunk = string;
+export opaque type Chunk = string;
 
 export function scheduleWork(callback: () => void) {
   callback();

packages/react-server-dom-relay/src/ReactServerStreamConfigFB.js

@@ -14,8 +14,8 @@ export type Destination = {
   error: mixed,
 };
 
-export type PrecomputedChunk = string;
-export type Chunk = string;
+export opaque type PrecomputedChunk = string;
+export opaque type Chunk = string;
 
 export function scheduleWork(callback: () => void) {
   // We don't schedule work in this model, and instead expect performWork to always be called repeatedly.

packages/react-server/src/ReactServerStreamConfigBrowser.js

@@ -10,7 +10,7 @@
 export type Destination = ReadableStreamController;
 
 export type PrecomputedChunk = Uint8Array;
-export type Chunk = Uint8Array;
+export opaque type Chunk = Uint8Array;
 
 export function scheduleWork(callback: () => void) {
   callback();

packages/react-server/src/ReactServerStreamConfigNode.js

@@ -17,7 +17,7 @@ interface MightBeFlushable {
 export type Destination = Writable & MightBeFlushable;
 
 export type PrecomputedChunk = Uint8Array;
-export type Chunk = string;
+export opaque type Chunk = string;
 
 export function scheduleWork(callback: () => void) {
   setImmediate(callback);