use .js.map extension for cached source maps

This is the more commonly used extension for source map files.

Fix the test which erroneously looked for `.js` files where it meant `.map`.

lib/caching-precompiler.js

@@ -59,7 +59,7 @@ CachingPrecompiler.prototype._factory = function (babelConfig, cacheDir) {
 		code = code.toString();
 		var options = buildOptions(filename, code);
 		var result = babel.transform(code, options);
-		var mapFile = path.join(cacheDir, hash + '.map');
+		var mapFile = path.join(cacheDir, hash + '.js.map');
 		fs.writeFileSync(mapFile, JSON.stringify(result.map));
 		return result.code;
 	};

lib/test-worker.js

@@ -72,7 +72,7 @@ installPrecompiler(function (filename) {
 	var precompiled = opts.precompiled[filename];
 
 	if (precompiled) {
-		sourceMapCache[filename] = path.join(cacheDir, precompiled + '.map');
+		sourceMapCache[filename] = path.join(cacheDir, precompiled + '.js.map');
 		return fs.readFileSync(path.join(cacheDir, precompiled + '.js'), 'utf8');
 	}
 

test/caching-precompiler.js

@@ -18,7 +18,7 @@ function endsWithJs(filename) {
 }
 
 function endsWithMap(filename) {
-	return /\.js$/.test(filename);
+	return /\.js\.map$/.test(filename);
 }
 
 sinon.spy(babel, 'transform');
@@ -50,7 +50,7 @@ test('adds files and source maps to the cache directory as needed', function (t)
 	var files = fs.readdirSync(tempDir);
 	t.is(files.length, 2);
 	t.is(files.filter(endsWithJs).length, 1, 'one .js file is saved to the cache');
-	t.is(files.filter(endsWithMap).length, 1, 'one .map file is saved to the cache');
+	t.is(files.filter(endsWithMap).length, 1, 'one .js.map file is saved to the cache');
 	t.end();
 });