Port katex, buildTree, parseTree to @flow. (#989)

src/Settings.js

@@ -6,7 +6,7 @@
 
 import utils from "./utils";
 
-type SettingsOptions = {
+export type SettingsOptions = {
     displayMode?: boolean;
     throwOnError?: boolean;
     errorColor?: string;

src/buildTree.js

@@ -1,3 +1,4 @@
+// @flow
 import buildHTML from "./buildHTML";
 import buildMathML from "./buildMathML";
 import buildCommon from "./buildCommon";
@@ -5,7 +6,14 @@ import Options from "./Options";
 import Settings from "./Settings";
 import Style from "./Style";
 
-const buildTree = function(tree, expression, settings) {
+import type ParseNode from "./ParseNode";
+import type domTree from "./domTree";
+
+const buildTree = function(
+    tree: ParseNode[],
+    expression: string,
+    settings: Settings,
+): domTree.span {
     settings = settings || new Settings({});
 
     let startStyle = Style.TEXT;

src/parseTree.js

@@ -1,3 +1,4 @@
+// @flow
 /**
  * Provides a single function for parsing an expression using a Parser
  * TODO(emily): Remove this
@@ -5,10 +6,13 @@
 
 import Parser from "./Parser";
 
+import type ParseNode from "./ParseNode";
+import type Settings from "./Settings";
+
 /**
  * Parses an expression using a Parser, then returns the parsed result.
  */
-const parseTree = function(toParse, settings) {
+const parseTree = function(toParse: string, settings: Settings): ParseNode[] {
     if (!(typeof toParse === 'string' || toParse instanceof String)) {
         throw new TypeError('KaTeX can only parse string typed expression');
     }