Add dark mode variant (#2279)

* Initial dark mode prototype

* Isolate dark mode logic to config extension

* Update fixtures

* Fix lint warnings

__tests__/darkMode.test.js

@@ -0,0 +1,240 @@
+import postcss from 'postcss'
+import tailwind from '../src/index'
+
+function run(input, config = {}) {
+  return postcss([tailwind({ experimental: { darkModeVariant: true }, ...config })]).process(
+    input,
+    { from: undefined }
+  )
+}
+
+test('dark mode variants cannot be generated without enabling the dark mode experiment', () => {
+  const input = `
+    @variants dark {
+      .text-red {
+        color: red;
+      }
+    }
+  `
+
+  expect.assertions(1)
+  return expect(run(input, { experimental: {} })).rejects.toThrow()
+})
+
+test('generating dark mode variants uses the media strategy by default', () => {
+  const input = `
+    @variants dark {
+      .text-red {
+        color: red;
+      }
+    }
+  `
+
+  const expected = `
+    .text-red {
+      color: red;
+    }
+    @media (prefers-color-scheme: dark) {
+      .dark\\:text-red {
+        color: red;
+      }
+    }
+  `
+
+  expect.assertions(2)
+
+  return run(input).then(result => {
+    expect(result.css).toMatchCss(expected)
+    expect(result.warnings().length).toBe(0)
+  })
+})
+
+test('dark mode variants can be generated using the class strategy', () => {
+  const input = `
+    @variants dark {
+      .text-red {
+        color: red;
+      }
+    }
+  `
+
+  const expected = `
+    .text-red {
+      color: red;
+    }
+    .dark .dark\\:text-red {
+      color: red;
+    }
+  `
+
+  expect.assertions(2)
+
+  return run(input, { dark: 'class' }).then(result => {
+    expect(result.css).toMatchCss(expected)
+    expect(result.warnings().length).toBe(0)
+  })
+})
+
+test('dark mode variants stack with other variants', () => {
+  const input = `
+    @variants responsive, dark, hover, focus {
+      .text-red {
+        color: red;
+      }
+    }
+  `
+
+  const expected = `
+    .text-red {
+      color: red;
+    }
+    .hover\\:text-red:hover {
+      color: red;
+    }
+    .focus\\:text-red:focus {
+      color: red;
+    }
+    @media (prefers-color-scheme: dark) {
+      .dark\\:text-red {
+        color: red;
+      }
+      .dark\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .dark\\:focus\\:text-red:focus {
+        color: red;
+      }
+    }
+    @media (min-width: 500px) {
+      .sm\\:text-red {
+        color: red;
+      }
+      .sm\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .sm\\:focus\\:text-red:focus {
+        color: red;
+      }
+      @media (prefers-color-scheme: dark) {
+        .sm\\:dark\\:text-red {
+          color: red;
+        }
+        .sm\\:dark\\:hover\\:text-red:hover {
+          color: red;
+        }
+        .sm\\:dark\\:focus\\:text-red:focus {
+          color: red;
+        }
+      }
+    }
+    @media (min-width: 800px) {
+      .lg\\:text-red {
+        color: red;
+      }
+      .lg\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .lg\\:focus\\:text-red:focus {
+        color: red;
+      }
+      @media (prefers-color-scheme: dark) {
+        .lg\\:dark\\:text-red {
+          color: red;
+        }
+        .lg\\:dark\\:hover\\:text-red:hover {
+          color: red;
+        }
+        .lg\\:dark\\:focus\\:text-red:focus {
+          color: red;
+        }
+      }
+    }
+  `
+
+  expect.assertions(2)
+
+  return run(input, { theme: { screens: { sm: '500px', lg: '800px' } } }).then(result => {
+    expect(result.css).toMatchCss(expected)
+    expect(result.warnings().length).toBe(0)
+  })
+})
+
+test('dark mode variants stack with other variants when using the class strategy', () => {
+  const input = `
+    @variants responsive, dark, hover, focus {
+      .text-red {
+        color: red;
+      }
+    }
+  `
+
+  const expected = `
+    .text-red {
+      color: red;
+    }
+    .hover\\:text-red:hover {
+      color: red;
+    }
+    .focus\\:text-red:focus {
+      color: red;
+    }
+    .dark .dark\\:text-red {
+      color: red;
+    }
+    .dark .dark\\:hover\\:text-red:hover {
+      color: red;
+    }
+    .dark .dark\\:focus\\:text-red:focus {
+      color: red;
+    }
+    @media (min-width: 500px) {
+      .sm\\:text-red {
+        color: red;
+      }
+      .sm\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .sm\\:focus\\:text-red:focus {
+        color: red;
+      }
+      .dark .sm\\:dark\\:text-red {
+        color: red;
+      }
+      .dark .sm\\:dark\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .dark .sm\\:dark\\:focus\\:text-red:focus {
+        color: red;
+      }
+    }
+    @media (min-width: 800px) {
+      .lg\\:text-red {
+        color: red;
+      }
+      .lg\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .lg\\:focus\\:text-red:focus {
+        color: red;
+      }
+      .dark .lg\\:dark\\:text-red {
+        color: red;
+      }
+      .dark .lg\\:dark\\:hover\\:text-red:hover {
+        color: red;
+      }
+      .dark .lg\\:dark\\:focus\\:text-red:focus {
+        color: red;
+      }
+    }
+  `
+
+  expect.assertions(2)
+
+  return run(input, { dark: 'class', theme: { screens: { sm: '500px', lg: '800px' } } }).then(
+    result => {
+      expect(result.css).toMatchCss(expected)
+      expect(result.warnings().length).toBe(0)
+    }
+  )
+})