diff --git a/packages/react-art/src/ReactARTHostConfig.js b/packages/react-art/src/ReactARTHostConfig.js
index 436bdc50859b8..48e4d9c102649 100644
--- a/packages/react-art/src/ReactARTHostConfig.js
+++ b/packages/react-art/src/ReactARTHostConfig.js
@@ -248,8 +248,6 @@ export function appendInitialChild(parentInstance, child) {
   if (typeof child === 'string') {
     // Noop for string children of Text (eg <Text>{'foo'}{'bar'}</Text>)
     throw new Error('Text children should already be flattened.');
-
-    return;
   }
 
   child.inject(parentInstance);
diff --git a/packages/react-dom/src/client/ReactDOMTextarea.js b/packages/react-dom/src/client/ReactDOMTextarea.js
index b98ec8d55b7cf..6834b16e4e97c 100644
--- a/packages/react-dom/src/client/ReactDOMTextarea.js
+++ b/packages/react-dom/src/client/ReactDOMTextarea.js
@@ -104,8 +104,6 @@ export function initWrapperState(element: Element, props: Object) {
         }
 
         if (isArray(children)) {
-          // FIXME: Review this condition before merging
-          // Should be equivalent to: !(children.length <= 1)
           if (children.length > 1) {
             throw new Error('<textarea> can only have at most one child.');
           }
diff --git a/packages/react-dom/src/server/ReactDOMServerFormatConfig.js b/packages/react-dom/src/server/ReactDOMServerFormatConfig.js
index 697808abbd032..dc129ef996c02 100644
--- a/packages/react-dom/src/server/ReactDOMServerFormatConfig.js
+++ b/packages/react-dom/src/server/ReactDOMServerFormatConfig.js
@@ -896,8 +896,6 @@ function pushStartTextArea(
     }
 
     if (isArray(children)) {
-      // FIXME: Review this condition before merging
-      // Should be equivalent to: !(children.length <= 1)
       if (children.length > 1) {
         throw new Error('<textarea> can only have at most one child.');
       }
diff --git a/packages/react-dom/src/server/ReactPartialRenderer.js b/packages/react-dom/src/server/ReactPartialRenderer.js
index eaecbd046676b..d1e0ff3b7b65e 100644
--- a/packages/react-dom/src/server/ReactPartialRenderer.js
+++ b/packages/react-dom/src/server/ReactPartialRenderer.js
@@ -976,8 +976,6 @@ class ReactDOMServerRenderer {
         } catch (err) {
           if (err != null && typeof err.then === 'function') {
             if (enableSuspenseServerRenderer) {
-              // FIXME: Review this condition before merging
-              // Should be equivalent to: !(this.suspenseDepth > 0)
               if (this.suspenseDepth <= 0) {
                 throw new Error(
                   // TODO: include component name. This is a bit tricky with current factoring.
@@ -1477,8 +1475,6 @@ class ReactDOMServerRenderer {
           }
 
           if (isArray(textareaChildren)) {
-            // FIXME: Review this condition before merging
-            // Should be equivalent to: !(textareaChildren.length <= 1)
             if (textareaChildren.length > 1) {
               throw new Error('<textarea> can only have at most one child.');
             }
diff --git a/packages/react-dom/src/server/ReactPartialRendererHooks.js b/packages/react-dom/src/server/ReactPartialRendererHooks.js
index 19d1dccc38e78..6e2b715382a0a 100644
--- a/packages/react-dom/src/server/ReactPartialRendererHooks.js
+++ b/packages/react-dom/src/server/ReactPartialRendererHooks.js
@@ -424,8 +424,6 @@ function dispatchAction<A>(
   queue: UpdateQueue<A>,
   action: A,
 ) {
-  // FIXME: Review this condition before merging
-  // Should be equivalent to: !(numberOfReRenders < RE_RENDER_LIMIT)
   if (numberOfReRenders >= RE_RENDER_LIMIT) {
     throw new Error(
       'Too many re-renders. React limits the number of renders to prevent ' +
diff --git a/packages/react-dom/src/server/ReactThreadIDAllocator.js b/packages/react-dom/src/server/ReactThreadIDAllocator.js
index c35a1b62873f2..fabf9fef98843 100644
--- a/packages/react-dom/src/server/ReactThreadIDAllocator.js
+++ b/packages/react-dom/src/server/ReactThreadIDAllocator.js
@@ -24,8 +24,6 @@ function growThreadCountAndReturnNextAvailable() {
   const oldSize = oldArray.length;
   const newSize = oldSize * 2;
 
-  // FIXME: Review this condition before merging
-  // Should be equivalent to: !(newSize <= 0x10000)
   if (newSize > 0x10000) {
     throw new Error(
       'Maximum number of concurrent React renderers exceeded. ' +
diff --git a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/UIManager.js b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/UIManager.js
index 05914344fcc01..346c4aa45c5f5 100644
--- a/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/UIManager.js
+++ b/packages/react-native-renderer/src/__mocks__/react-native/Libraries/ReactPrivate/UIManager.js
@@ -40,8 +40,6 @@ function insertSubviewAtIndex(parent, child, index) {
     );
   }
 
-  // FIXME: Review this condition before merging
-  // Should be equivalent to: !(0 <= index && index <= parentInfo.children.length)
   if (0 > index || index > parentInfo.children.length) {
     throw new Error(
       `Invalid index ${index} for children ${parentInfo.children}`,
@@ -57,8 +55,6 @@ function removeChild(parent, child) {
   const childInfo = views.get(child);
   const index = parentInfo.children.indexOf(child);
 
-  // FIXME: Review this condition before merging
-  // Should be equivalent to: !(index >= 0)
   if (index < 0) {
     throw new Error(`Missing view ${child} during removal`);
   }
diff --git a/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js b/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js
index 57797d8ff616a..3737d2c073b61 100644
--- a/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js
+++ b/packages/react-native-renderer/src/legacy-events/EventPluginRegistry.js
@@ -44,8 +44,6 @@ function recomputePluginOrdering(): void {
     const pluginModule = namesToPlugins[pluginName];
     const pluginIndex = eventPluginOrder.indexOf(pluginName);
 
-    // FIXME: Review this condition before merging
-    // Should be equivalent to: !(pluginIndex > -1)
     if (pluginIndex <= -1) {
       throw new Error(
         'EventPluginRegistry: Cannot inject event plugins that do not exist in ' +
diff --git a/packages/react-reconciler/src/ReactChildFiber.new.js b/packages/react-reconciler/src/ReactChildFiber.new.js
index 486d44a061b12..824600459c91b 100644
--- a/packages/react-reconciler/src/ReactChildFiber.new.js
+++ b/packages/react-reconciler/src/ReactChildFiber.new.js
@@ -169,6 +169,8 @@ function coerceRef(
             'bug in React. Please file an issue.',
         );
       }
+      // Assigning this to a const so Flow knows it won't change in the closure
+      const resolvedInst = inst;
 
       const stringRef = '' + mixedRef;
       // Check if previous string ref matches new string ref
@@ -181,10 +183,10 @@ function coerceRef(
         return current.ref;
       }
       const ref = function(value) {
-        let refs = inst.refs;
+        let refs = resolvedInst.refs;
         if (refs === emptyRefsObject) {
           // This is a lazy pooled frozen object, so we need to initialize.
-          refs = inst.refs = {};
+          refs = resolvedInst.refs = {};
         }
         if (value === null) {
           delete refs[stringRef];
diff --git a/packages/react-reconciler/src/ReactChildFiber.old.js b/packages/react-reconciler/src/ReactChildFiber.old.js
index 7bf6cee1be967..ebd40bb23ce10 100644
--- a/packages/react-reconciler/src/ReactChildFiber.old.js
+++ b/packages/react-reconciler/src/ReactChildFiber.old.js
@@ -169,6 +169,8 @@ function coerceRef(
             'bug in React. Please file an issue.',
         );
       }
+      // Assigning this to a const so Flow knows it won't change in the closure
+      const resolvedInst = inst;
 
       const stringRef = '' + mixedRef;
       // Check if previous string ref matches new string ref
@@ -181,10 +183,10 @@ function coerceRef(
         return current.ref;
       }
       const ref = function(value) {
-        let refs = inst.refs;
+        let refs = resolvedInst.refs;
         if (refs === emptyRefsObject) {
           // This is a lazy pooled frozen object, so we need to initialize.
-          refs = inst.refs = {};
+          refs = resolvedInst.refs = {};
         }
         if (value === null) {
           delete refs[stringRef];
diff --git a/packages/react-reconciler/src/ReactFiberHooks.new.js b/packages/react-reconciler/src/ReactFiberHooks.new.js
index ab612fba810b2..2a7e0e3c6f5e3 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.new.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.new.js
@@ -435,8 +435,6 @@ export function renderWithHooks<Props, SecondArg>(
     do {
       didScheduleRenderPhaseUpdateDuringThisPass = false;
 
-      // FIXME: Review this condition before merging
-      // Should be equivalent to: !(numberOfReRenders < RE_RENDER_LIMIT)
       if (numberOfReRenders >= RE_RENDER_LIMIT) {
         throw new Error(
           'Too many re-renders. React limits the number of renders to prevent ' +
diff --git a/packages/react-reconciler/src/ReactFiberHooks.old.js b/packages/react-reconciler/src/ReactFiberHooks.old.js
index b62512dad3ef6..41221a3ef7544 100644
--- a/packages/react-reconciler/src/ReactFiberHooks.old.js
+++ b/packages/react-reconciler/src/ReactFiberHooks.old.js
@@ -435,8 +435,6 @@ export function renderWithHooks<Props, SecondArg>(
     do {
       didScheduleRenderPhaseUpdateDuringThisPass = false;
 
-      // FIXME: Review this condition before merging
-      // Should be equivalent to: !(numberOfReRenders < RE_RENDER_LIMIT)
       if (numberOfReRenders >= RE_RENDER_LIMIT) {
         throw new Error(
           'Too many re-renders. React limits the number of renders to prevent ' +
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.new.js b/packages/react-reconciler/src/ReactFiberReconciler.new.js
index 577f981ec664a..814efea10be82 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.new.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.new.js
@@ -154,10 +154,9 @@ function findHostInstance(component: Object): PublicInstance | null {
     if (typeof component.render === 'function') {
       throw new Error('Unable to find node on an unmounted component.');
     } else {
+      const keys = Object.keys(component).join(',');
       throw new Error(
-        `Argument appears to not be a ReactComponent. Keys: ${Object.keys(
-          component,
-        )}`,
+        `Argument appears to not be a ReactComponent. Keys: ${keys}`,
       );
     }
   }
@@ -178,10 +177,9 @@ function findHostInstanceWithWarning(
       if (typeof component.render === 'function') {
         throw new Error('Unable to find node on an unmounted component.');
       } else {
+        const keys = Object.keys(component).join(',');
         throw new Error(
-          `Argument appears to not be a ReactComponent. Keys: ${Object.keys(
-            component,
-          )}`,
+          `Argument appears to not be a ReactComponent. Keys: ${keys}`,
         );
       }
     }
diff --git a/packages/react-reconciler/src/ReactFiberReconciler.old.js b/packages/react-reconciler/src/ReactFiberReconciler.old.js
index f4c9df7683dd2..31a0b1fff6868 100644
--- a/packages/react-reconciler/src/ReactFiberReconciler.old.js
+++ b/packages/react-reconciler/src/ReactFiberReconciler.old.js
@@ -154,10 +154,9 @@ function findHostInstance(component: Object): PublicInstance | null {
     if (typeof component.render === 'function') {
       throw new Error('Unable to find node on an unmounted component.');
     } else {
+      const keys = Object.keys(component).join(',');
       throw new Error(
-        `Argument appears to not be a ReactComponent. Keys: ${Object.keys(
-          component,
-        )}`,
+        `Argument appears to not be a ReactComponent. Keys: ${keys}`,
       );
     }
   }
@@ -178,10 +177,9 @@ function findHostInstanceWithWarning(
       if (typeof component.render === 'function') {
         throw new Error('Unable to find node on an unmounted component.');
       } else {
+        const keys = Object.keys(component).join(',');
         throw new Error(
-          `Argument appears to not be a ReactComponent. Keys: ${Object.keys(
-            component,
-          )}`,
+          `Argument appears to not be a ReactComponent. Keys: ${keys}`,
         );
       }
     }
diff --git a/packages/react-reconciler/src/ReactTestSelectors.js b/packages/react-reconciler/src/ReactTestSelectors.js
index 944c713881ea6..adadc8ed73e80 100644
--- a/packages/react-reconciler/src/ReactTestSelectors.js
+++ b/packages/react-reconciler/src/ReactTestSelectors.js
@@ -183,11 +183,7 @@ function matchSelector(fiber: Fiber, selector: Selector): boolean {
       }
       break;
     default:
-      if (!null) {
-        throw new Error(`Invalid selector type ${selector} specified.`);
-      }
-
-      break;
+      throw new Error('Invalid selector type specified.');
   }
 
   return false;
@@ -207,14 +203,8 @@ function selectorToString(selector: Selector): string | null {
     case TEST_NAME_TYPE:
       return `[data-testname="${((selector: any): TestNameSelector).value}"]`;
     default:
-      if (!null) {
-        throw new Error(`Invalid selector type ${selector} specified.`);
-      }
-
-      break;
+      throw new Error('Invalid selector type specified.');
   }
-
-  return null;
 }
 
 function findPaths(root: Fiber, selectors: Array<Selector>): Array<Fiber> {
diff --git a/packages/react-server/src/ReactFizzHooks.js b/packages/react-server/src/ReactFizzHooks.js
index 21b30c966d139..6370f61b37735 100644
--- a/packages/react-server/src/ReactFizzHooks.js
+++ b/packages/react-server/src/ReactFizzHooks.js
@@ -407,8 +407,6 @@ function dispatchAction<A>(
   queue: UpdateQueue<A>,
   action: A,
 ) {
-  // FIXME: Review this condition before merging
-  // Should be equivalent to: !(numberOfReRenders < RE_RENDER_LIMIT)
   if (numberOfReRenders >= RE_RENDER_LIMIT) {
     throw new Error(
       'Too many re-renders. React limits the number of renders to prevent ' +
diff --git a/scripts/error-codes/codes.json b/scripts/error-codes/codes.json
index d11735986235a..cc5109bd2b172 100644
--- a/scripts/error-codes/codes.json
+++ b/scripts/error-codes/codes.json
@@ -351,7 +351,7 @@
   "362": "Could not find React container within specified host subtree.",
   "363": "Test selector API is not supported by this renderer.",
   "364": "Invalid host root specified. Should be either a React container or a node with a testname attribute.",
-  "365": "Invalid selector type %s specified.",
+  "365": "Invalid selector type specified.",
   "366": "ReactDOM.createEventHandle: setListener called on an target that did not have a corresponding root. This is likely a bug in React.",
   "367": "ReactDOM.createEventHandle: setListener called on an element target that is not managed by React. Ensure React rendered the DOM element.",
   "368": "ReactDOM.createEventHandle: setListener called on an invalid target. Provide a valid EventTarget or an element managed by React.",