Port spacingData and unicodeSymbols to @flow. (#1195)

* Port spacingData and unicodeSymbols to @flow.

* Review fixes.

* Made Measurement an exact object type.

src/spacingData.js

@@ -1,22 +1,40 @@
+// @flow
 /**
  * Describes spaces between different classes of atoms.
  */
+import type {Measurement} from "./units";
 
-const thinspace = {
+const thinspace: Measurement = {
     number: 3,
     unit: "mu",
 };
-const mediumspace = {
+const mediumspace: Measurement = {
     number: 4,
     unit: "mu",
 };
-const thickspace = {
+const thickspace: Measurement = {
     number: 5,
     unit: "mu",
 };
 
+// Making the type below exact with all optional fields doesn't work due to
+// - https://github.com/facebook/flow/issues/4582
+// - https://github.com/facebook/flow/issues/5688
+// However, since *all* fields are optional, $Shape<> works as suggested in 5688
+// above.
+export type Spacings = $Shape<{
+    mord: Measurement,
+    mop: Measurement,
+    mbin: Measurement,
+    mrel: Measurement,
+    mopen: Measurement,
+    mclose: Measurement,
+    mpunct: Measurement,
+    minner: Measurement,
+}> & {};
+
 // Spacing relationships for display and text styles
-export const spacings = {
+export const spacings: {[$Keys<Spacings>]: Spacings} = {
     mord: {
         mop: thinspace,
         mbin: mediumspace,
@@ -69,7 +87,7 @@ export const spacings = {
 };
 
 // Spacing relationships for script and scriptscript styles
-export const tightSpacings = {
+export const tightSpacings: {[$Keys<Spacings>]: Spacings} = {
     mord: {
         mop: thinspace,
     },

src/unicodeMake.js

@@ -7,6 +7,7 @@
 
 const accents = require('./unicodeAccents');
 
+console.log("// @flow");
 console.log("// This file is GENERATED by unicodeMake.js. DO NOT MODIFY.");
 console.log("");
 

src/unicodeSymbols.js

@@ -1,3 +1,4 @@
+// @flow
 // This file is GENERATED by unicodeMake.js. DO NOT MODIFY.
 
 export default {

src/units.js

@@ -37,7 +37,7 @@ const relativeUnit = {
     "mu": true,
 };
 
-export type Measurement = {number: number, unit: string};
+export type Measurement = {| number: number, unit: string |};
 
 /**
  * Determine whether the specified unit (either a string defining the unit