fix plugin-sass partial loading

plugins/plugin-sass/plugin.js

@@ -84,6 +84,12 @@ module.exports = function sassPlugin(_, {native, compilerOptions = {}} = {}) {
     async load({filePath, isDev}) {
       const fileExt = path.extname(filePath);
       const contents = fs.readFileSync(filePath, 'utf8');
+
+      // Sass partials should never be loaded directly, return nothing to ignore.
+      if (path.basename(filePath).startsWith('_')) {
+        return;
+      }
+
       // During development, we need to track changes to Sass dependencies.
       if (isDev) {
         const sassImports = scanSassImports(contents, filePath, fileExt);

plugins/plugin-sass/test/plugin.test.js

@@ -2,7 +2,7 @@ const plugin = require('../plugin.js');
 const path = require('path');
 
 const pathToSassApp = path.join(__dirname, 'fixtures/sass/App.sass');
-const pathToSassBase = path.join(__dirname, 'fixtures/sass/base.sass');
+const pathToSassBase = path.join(__dirname, 'fixtures/sass/_base.sass');
 const pathToScssApp = path.join(__dirname, 'fixtures/scss/App.scss');
 const pathToBadCode = path.join(__dirname, 'fixtures/bad/bad.scss');
 
@@ -15,6 +15,14 @@ describe('plugin-sass', () => {
     expect(scssResult).toMatchSnapshot('App.scss');
   });
 
+  test('returns undefined when a sass partial is loaded directly', async () => {
+    const p = plugin(null, {});
+    const devResult = await p.load({filePath: pathToSassBase, isDev: false});
+    expect(devResult).toEqual(undefined);
+    const prodResult = await p.load({filePath: pathToSassBase, isDev: true});
+    expect(prodResult).toEqual(undefined);
+  });
+
   test('throws an error when stderr output is returned', async () => {
     const p = plugin(null, {});
     await expect(p.load({filePath: pathToBadCode, isDev: false})).rejects.toThrow(