Port symbol.js to @flow. (#858)

* Port symbol.js to @flow.

* Add a `Mode` @flow type and use it in `ParseNode`.

src/ParseNode.js

@@ -1,5 +1,6 @@
 // @flow
 import {LexerInterface, Token} from "./Token";
+import type {Mode} from "./types";
 
 /**
  * The resulting parse tree nodes of the parse tree.
@@ -12,7 +13,7 @@ import {LexerInterface, Token} from "./Token";
 export default class ParseNode {
     type: *;
     value: *;
-    mode: *;
+    mode: Mode;
     // TODO: We should combine these to ({lexer, start, end}|void) as they
     // should all exist together or not exist at all. That way, only a single
     // void check needs to be done to see if we have metadata.
@@ -23,7 +24,7 @@ export default class ParseNode {
     constructor(
         type: string,       // type of node, like e.g. "ordgroup"
         value: mixed,       // type-specific representation of the node
-        mode: string,       // parse mode in action for this node, "math" or "text"
+        mode: Mode,         // parse mode in action for this node, "math" or "text"
         firstToken?: Token, // first token of the input for this node,
                             // will omit position information if unset
         lastToken?: Token,  // last token of the input for this node,

src/symbols.js

@@ -1,3 +1,4 @@
+// @flow
 /**
  * This file holds a list of all no-argument functions and single-character
  * symbols (like 'a' or ';').
@@ -16,20 +17,33 @@
  * accepted in (e.g. "math" or "text").
  */
 
-module.exports = {
-    math: {},
-    text: {},
+import type {Mode} from "./types";
+
+type Font = "main" | "ams";
+type Group =
+    "accent" | "bin" | "close" | "inner" | "mathord" | "op" | "open" | "punct" |
+    "rel" | "spacing" | "textord";
+type CharInfoMap = {[string]: {font: Font, group: Group, replace: ?string}};
+
+const symbols: {[Mode]: CharInfoMap} = {
+    "math": {},
+    "text": {},
 };
+export default symbols;
 
-function defineSymbol(mode, font, group, replace, name, acceptUnicodeChar) {
-    module.exports[mode][name] = {
-        font: font,
-        group: group,
-        replace: replace,
-    };
+/** `acceptUnicodeChar = true` is only applicable if `replace` is set. */
+function defineSymbol(
+    mode: Mode,
+    font: Font,
+    group: Group,
+    replace: ?string,
+    name: string,
+    acceptUnicodeChar?: boolean,
+) {
+    symbols[mode][name] = {font, group, replace};
 
-    if (acceptUnicodeChar) {
-        module.exports[mode][replace] = module.exports[mode][name];
+    if (acceptUnicodeChar && replace) {
+        symbols[mode][replace] = symbols[mode][name];
     }
 }
 

src/types.js

@@ -0,0 +1,9 @@
+// @flow
+
+/**
+ * This file consists only of basic flow types used in multiple places.
+ * For types with javascript, create separate files by themselves.
+ */
+
+export type Mode = "math" | "text";
+