Also infer type from const initializer

Infer the type for statements like:

```js
const x = 42;
```

same as:

```js
const x: number = 42;
```

Author: arv

lib/infer/type.js

@@ -5,6 +5,12 @@ var finders = require('./finders'),
   flowDoctrine = require('../flow_doctrine'),
   t = require('babel-types');
 
+var constTypeMapping = {
+  'BooleanLiteral': {type: 'BooleanTypeAnnotation'},
+  'NumericLiteral': {type: 'NumberTypeAnnotation'},
+  'StringLiteral': {type: 'StringTypeAnnotation'}
+};
+
 /**
  * Infers type tags by using Flow type annotations
  *
@@ -27,6 +33,9 @@ module.exports = function () {
     switch (n.type) {
     case 'VariableDeclarator':
       type = n.id.typeAnnotation;
+      if (!type && comment.kind === 'constant') {
+        type = constTypeMapping[n.init.type];
+      }
       break;
     case 'ClassProperty':
       type = n.typeAnnotation;

test/lib/infer/type.js

@@ -107,5 +107,29 @@ test('inferType', function (t) {
     type: 'NameExpression'
   });
 
+  t.deepEqual(evaluate(
+    '/** */' +
+    'const x = 42;'
+  ).type, {
+    name: 'number',
+    type: 'NameExpression'
+  });
+
+  t.deepEqual(evaluate(
+    '/** */' +
+    'const x = "abc";'
+  ).type, {
+    name: 'string',
+    type: 'NameExpression'
+  });
+
+  t.deepEqual(evaluate(
+    '/** */' +
+    'const x = true;'
+  ).type, {
+    name: 'boolean',
+    type: 'NameExpression'
+  });
+
   t.end();
 });