ensure import paths are case-sensitive (#590)

src/utils/defaults.js

@@ -1,5 +1,5 @@
-import { isFile, readFileSync } from './fs.js';
-import { dirname, isAbsolute, resolve } from './path.js';
+import { isFile, readdirSync, readFileSync } from './fs.js';
+import { basename, dirname, isAbsolute, resolve } from './path.js';
 import { blank } from './object.js';
 
 export function load ( id ) {
@@ -7,10 +7,11 @@ export function load ( id ) {
 }
 
 function addJsExtensionIfNecessary ( file ) {
-	if ( isFile( file ) ) return file;
+	const name = basename( file );
+	const files = readdirSync( dirname( file ) );
 
-	file += '.js';
-	if ( isFile( file ) ) return file;
+	if ( ~files.indexOf( name ) && isFile( file ) ) return file;
+	if ( ~files.indexOf( `${name}.js` ) && isFile( `${file}.js` ) ) return `${file}.js`;
 
 	return null;
 }

test/function/paths-are-case-sensitive/Foo.js

@@ -0,0 +1,3 @@
+export default function () {
+	assert.ok( false );
+}

test/function/paths-are-case-sensitive/_config.js

@@ -0,0 +1,8 @@
+var assert = require( 'assert' );
+
+module.exports = {
+	description: 'insists on correct casing for imports',
+	error: function ( err ) {
+		assert.ok( /Could not resolve/.test( err.message ) );
+	}
+};

test/function/paths-are-case-sensitive/main.js

@@ -0,0 +1,3 @@
+import foo from './foo.js';
+
+foo();