flow: Use typeof TextInput when we want the type, not the value.

We add `typeof` in several places to address this Flow error that
starts appearing at the RN and Flow upgrade:

```
Cannot use `TextInput` as a type. A name can be used as a type only
if it refers to a type definition, an interface definition, or a
class definition. To get the type of a non-class value, use
`typeof`.
```

I'm not sure if this is due to the Flow upgrade or to changes React
Native made to `TextInput`, or both. Several changes to `TextInput`
are announced in the RN changelog [1], including three that are
breaking, but I haven't been able to identify any in particular that
would start giving us that error.

With that dealt with, we also get this new error on calling various
methods on the instance stored at a `TextInput` ref (e.g.,
`textInputRef.current.focus()`):

```
Cannot call textInputRef.current.focus because:
 • Either property focus is missing in AbstractComponent [1].
 • Or property focus is missing in object type [2].
```

At first, I thought something was wrong with how we're annotating
the variable storing the ref, or that Flow didn't fully understand
the `React.createRef` API (we started using that in a recent commit
before the main upgrade commit). But rather, it seems to be an issue
that's known to occur at RN v0.61.1, and which didn't occur on
`master` as of 2020-04-06 [2]. Checking commits around that date in
`react-native`, I'm pretty sure we'll have a fix in RN v0.63
(zulip#4245).

[1] https://github.com/react-native-community/releases/blob/master/CHANGELOG.md#0620
[2] See point 2 at
    facebook/react-native#28459 (comment).

src/common/Input.js

@@ -8,10 +8,10 @@ import type { ThemeData } from '../styles';
 import { ThemeContext, HALF_COLOR, BORDER_COLOR } from '../styles';
 
 export type Props = $ReadOnly<{|
-  // TextInput's definition changes across the RN v0.61 -> v0.62
-  // upgrade; we'll handle that change after the upgrade.
+  // See point 2 at
+  // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
   // $FlowFixMe
-  ...$PropertyType<TextInput, 'props'>,
+  ...$PropertyType<typeof TextInput, 'props'>,
   placeholder: LocalizableText,
   onChangeText?: (text: string) => void,
   textInputRef?: React$Ref<typeof TextInput>,

src/common/InputWithClearButton.js

@@ -32,10 +32,7 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
     canBeCleared: false,
     text: '',
   };
-  // TextInput's definition changes across the RN v0.61 -> v0.62
-  // upgrade; we'll handle that change after the upgrade.
-  // $FlowFixMe
-  textInputRef = React.createRef<TextInput>();
+  textInputRef = React.createRef<typeof TextInput>();
 
   handleChangeText = (text: string) => {
     this.setState({
@@ -50,6 +47,9 @@ export default class InputWithClearButton extends PureComponent<Props, State> {
   handleClear = () => {
     this.handleChangeText('');
     if (this.textInputRef.current) {
+      // See point 2 at
+      // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
+      // $FlowFixMe
       this.textInputRef.current.clear();
     }
   };

src/common/SmartUrlInput.js

@@ -66,15 +66,15 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
   state = {
     value: '',
   };
-  // TextInput's definition changes across the RN v0.61 -> v0.62
-  // upgrade; we'll handle that change after the upgrade.
-  // $FlowFixMe
-  textInputRef = React.createRef<TextInput>();
+  textInputRef = React.createRef<typeof TextInput>();
   focusListener: void | NavigationEventSubscription;
 
   componentDidMount() {
     this.focusListener = this.props.navigation.addListener('didFocus', () => {
       if (this.textInputRef.current) {
+        // See point 2 at
+        // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
+        // $FlowFixMe
         this.textInputRef.current.focus();
       }
     });
@@ -96,9 +96,13 @@ export default class SmartUrlInput extends PureComponent<Props, State> {
   urlPress = () => {
     const { textInputRef } = this;
     if (textInputRef.current) {
+      // See point 2 at
+      // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
+      // $FlowFixMe
       textInputRef.current.blur();
       setTimeout(() => {
         if (textInputRef.current) {
+          // $FlowFixMe - same as above
           textInputRef.current.focus();
         }
       }, 100);

src/compose/ComposeBox.js

@@ -105,16 +105,16 @@ function randomInt(min, max) {
   return Math.floor(Math.random() * (max - min + 1)) + min;
 }
 
-// TextInput's definition changes across the RN v0.61 -> v0.62
-// upgrade; we'll handle that change after the upgrade.
-// $FlowFixMe
-export const updateTextInput = (textInput: TextInput | null, text: string): void => {
+export const updateTextInput = (textInput: typeof TextInput | null, text: string): void => {
   if (textInput === null) {
     // Depending on the lifecycle events this function is called from,
     // this might not be set yet.
     return;
   }
 
+  // See point 2 at
+  // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
+  // $FlowFixMe
   textInput.setNativeProps({ text });
 
   if (text.length === 0 && TextInputReset) {
@@ -128,12 +128,8 @@ class ComposeBox extends PureComponent<Props, State> {
   static contextType = ThemeContext;
   context: ThemeData;
 
-  // TextInput's definition changes across the RN v0.61 -> v0.62
-  // upgrade; we'll remove these fixmes after we take that upgrade.
-  // $FlowFixMe
-  messageInputRef = React.createRef<TextInput>();
-  // $FlowFixMe
-  topicInputRef = React.createRef<TextInput>();
+  messageInputRef = React.createRef<typeof TextInput>();
+  topicInputRef = React.createRef<typeof TextInput>();
 
   // TODO: Type-check this, once we've adjusted our `react-redux`
   // wrapper to do the right thing. It should be
@@ -344,6 +340,9 @@ class ComposeBox extends PureComponent<Props, State> {
     }
     completeEditMessage();
     if (this.messageInputRef.current !== null) {
+      // See point 2 at
+      // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
+      // $FlowFixMe
       this.messageInputRef.current.blur();
     }
   };
@@ -358,6 +357,9 @@ class ComposeBox extends PureComponent<Props, State> {
       this.setMessageInputValue(message);
       this.setTopicInputValue(topic);
       if (this.messageInputRef.current !== null) {
+        // See point 2 at
+        // https://github.com/facebook/react-native/issues/28459#issuecomment-609957836
+        // $FlowFixMe
         this.messageInputRef.current.focus();
       }
     }