[Float] add support for scripts and other enhancements (facebook#25480)

* float enhance!!!

Support preinit as script
Support resources from async scripts
Support saving the precedence place when rendering the shell

There was a significant change to the flushing order of resources which follows the general principal of...
1. stuff that blocks display
2. stuff that we know will be used
3. stuff that was explicitly preloaded

As a consequence if you preinit a style now it won't automatically flush in the shell unless you actually depend on it in your tree. To avoid races with precedence order we now emit a tag that saves the place amongst the precedence hierarchy so late insertions still end up where they were intended

There is also a novel hydration pathway for certain tags. If you render an async script with an onLoad or onError it will always treat it like an insertion rather than a hydration.

* restore preinit style flushing behavior and nits

packages/react-dom-bindings/src/client/ReactDOMComponentTree.js

@@ -7,7 +7,11 @@
  * @flow
  */
 
-import type {FloatRoot, StyleResource} from './ReactDOMFloatClient';
+import type {
+  FloatRoot,
+  StyleResource,
+  ScriptResource,
+} from './ReactDOMFloatClient';
 import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
 import type {ReactScopeInstance} from 'shared/ReactTypes';
 import type {
@@ -48,7 +52,7 @@ const internalContainerInstanceKey = '__reactContainer$' + randomKey;
 const internalEventHandlersKey = '__reactEvents$' + randomKey;
 const internalEventHandlerListenersKey = '__reactListeners$' + randomKey;
 const internalEventHandlesSetKey = '__reactHandles$' + randomKey;
-const internalRootNodeStylesSetKey = '__reactStyles$' + randomKey;
+const internalRootNodeResourcesKey = '__reactResources$' + randomKey;
 
 export function detachDeletedInstance(node: Instance): void {
   // TODO: This function is only called on host components. I don't think all of
@@ -278,10 +282,15 @@ export function doesTargetHaveEventHandle(
   return eventHandles.has(eventHandle);
 }
 
-export function getStylesFromRoot(root: FloatRoot): Map<string, StyleResource> {
-  let styles = (root: any)[internalRootNodeStylesSetKey];
-  if (!styles) {
-    styles = (root: any)[internalRootNodeStylesSetKey] = new Map();
+export function getResourcesFromRoot(
+  root: FloatRoot,
+): {styles: Map<string, StyleResource>, scripts: Map<string, ScriptResource>} {
+  let resources = (root: any)[internalRootNodeResourcesKey];
+  if (!resources) {
+    resources = (root: any)[internalRootNodeResourcesKey] = {
+      styles: new Map(),
+      scripts: new Map(),
+    };
   }
-  return styles;
+  return resources;
 }