Add PropTypes to existing React components

Test Plan:
Run `yarn start` and note that the app runs without any console
warnings, but that changing some of the PropTypes to introduce an error
actually does cause a warning. Upcoming: an automatic test for this.

wchargin-branch: add-proptypes

explorer/src/FileExplorer.js

@@ -1,7 +1,15 @@
 import React, { Component } from 'react';
+import PropTypes from 'prop-types';
 import {buildTree} from './commitUtils';
+import {propTypes as commitUtilsPropTypes} from './commitUtils';
 
 export class FileExplorer extends Component {
+  static propTypes = {
+    selectedPath: PropTypes.string,
+    onSelectPath: PropTypes.func.isRequired,
+    data: commitUtilsPropTypes.commitData.isRequired,
+  }
+
   render() {
     // within the FileExplorer, paths start with "./", outside they don't
     // which is hacky and should be cleaned up
@@ -31,6 +39,18 @@ export class FileExplorer extends Component {
 }
 
 class FileEntry extends Component {
+  static propTypes = {
+    name: PropTypes.string.isRequired,
+    path: PropTypes.string.isRequired,
+    alwaysExpand: PropTypes.bool.isRequired,
+
+    // The type for the tree is recursive, and is annoying to specify as
+    // a proptype. The Flow type definition is in commitUtils.js.
+    tree: PropTypes.object.isRequired,
+
+    selectedPath: PropTypes.string.isRequired,
+    onSelectPath: PropTypes.func.isRequired,
+  }
 
   constructor() {
     super();

explorer/src/UserExplorer.js

@@ -1,7 +1,19 @@
 import React, { Component } from 'react';
-import {userWeightForPath, commitWeight} from './commitUtils';
+import PropTypes from 'prop-types';
+import {
+  commitWeight,
+  propTypes as commitUtilsPropTypes,
+  userWeightForPath,
+} from './commitUtils';
 
 export class UserExplorer extends Component {
+  static propTypes = {
+    selectedPath: PropTypes.string.isRequired,
+    selectedUser: PropTypes.string,
+    onSelectUser: PropTypes.func.isRequired,
+    data: commitUtilsPropTypes.commitData.isRequired,
+  }
+
   render() {
     const weights = userWeightForPath(this.props.selectedPath, this.props.data, commitWeight);
     const sortedUserWeightTuples = Object.entries(weights).sort((a,b) => b[1] - a[1]);
@@ -16,11 +28,15 @@ export class UserExplorer extends Component {
   }
 }
 
+/**
+ * Record the cred earned by the user in a given scope.
+ */
 class UserEntry extends Component {
-  // Record the cred earned by the user in a given scope
+  static propTypes = {
-  // Props: 
+    userId: PropTypes.string.isRequired,
-  //  userId, string
+    weight: PropTypes.number.isRequired,
-  //  weight, number
+  }
+
   render() {
     return <div className="user-entry">
       <span> {this.props.userId} </span>

explorer/src/commitUtils.js

@@ -1,5 +1,7 @@
 // @flow
 
+import PropTypes from 'prop-types';
+
 type CommitData = {
   // TODO improve variable names
   fileToCommits: {[filename: string]: string[]};
@@ -7,6 +9,23 @@ type CommitData = {
   authors: string[];
 }
 
+export const propTypes = {
+  commitData: PropTypes.shape({
+    fileToCommits: PropTypes.objectOf(
+      PropTypes.arrayOf(PropTypes.string.isRequired).isRequired,
+    ).isRequired,
+    commits: PropTypes.objectOf(PropTypes.shape({
+      author: PropTypes.string.isRequired,
+      stats: PropTypes.objectOf(PropTypes.shape({
+        lines: PropTypes.number.isRequired,
+        insertions: PropTypes.number.isRequired,
+        deletions: PropTypes.number.isRequired,
+      }).isRequired).isRequired,
+    }).isRequired).isRequired,
+  }),
+};
+
+
 type Commit = {
   author: string;
   stats: {[filename: string]: FileStats};