correctly rewrite exported var declarations (fixes #484)

src/Module.js

@@ -440,7 +440,12 @@ export default class Module {
 					const declaration = this.declarations[ declarator.id.name ];
 
 					if ( declaration.exportName && declaration.isReassigned ) { // `var foo = ...` becomes `exports.foo = ...`
-						magicString.remove( statement.start, declarator.init ? declarator.start : statement.next );
+						if ( declarator.init ) {
+							magicString.overwrite( statement.start, declarator.init.start, `exports.${declaration.exportName} = ` );
+						} else {
+							magicString.remove( statement.start, declarator.init ? declarator.start : statement.next );
+						}
+
 						return;
 					}
 				}

test/function/disappearing-exported-value/_config.js

@@ -0,0 +1,9 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'exported values do not mysteriously disappear (#484)',
+	exports: function ( exports ) {
+		assert.equal( exports.exportedAnswer, 42 );
+		assert.equal( exports.foo(), 42 );
+	}
+};

test/function/disappearing-exported-value/answer.js

@@ -0,0 +1,2 @@
+var answer = 42;
+export { answer };

test/function/disappearing-exported-value/main.js

@@ -0,0 +1,8 @@
+import { answer as importedAnswer } from './answer.js';
+export { answer as exportedAnswer } from './answer.js';
+
+export function foo () {
+	var value;
+	value = importedAnswer;
+	return value;
+}