feat: create indigo gradients

Closes #128

CHANGELOG.md

@@ -22,6 +22,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `GradOf` constructors: `green`, `greenAccent`; `lightGreen`,
   `lighGreenAccent`; `lime`, `limeAccent`; `teal`, `tealAccent`.
 
+[Indigo gradients](https://github.com/dartoos-dev/eo_color/issues/128):
+
+- `GradOf` constructors: `indigo`, `indigoAccent`.
+
 ## [2.1.0] - 2021-11-08
 
 ### Added

lib/src/material/grad_of.dart

@@ -153,6 +153,21 @@ class GradOf extends GradientSwatch {
         ); // coverage:ignore-line
   // coverage:ignore-end
 
+  /// A gradient of ten shades of indigo.
+  ///
+  /// See also: [Indigos].
+  const GradOf.indigo({bool growable = false})
+      : super(const Indigos(), growable: growable); // coverage:ignore-line
+  /// A gradient of four shades of indigo accent.
+  ///
+  /// See also: [IndigoAccents].
+  const GradOf.indigoAccent({bool growable = false})
+      // coverage:ignore-start
+      : super(
+          const IndigoAccents(),
+          growable: growable,
+        ); // coverage:ignore-line
+  // coverage:ignore-end
   /// A gradient of ten shades of green.
   ///
   /// See also: [Greens].

test/material/grad_of_test.dart

@@ -322,6 +322,36 @@ void main() {
         expect(growable.length, prevLength + 1);
       });
     });
+    group('indigo:', () {
+      final indigoGrad = const GradOf.indigo().colors;
+      test('gradient', () {
+        expect(indigoGrad, List.of(const Indigos().colors));
+      });
+      test('fixed-length list', () {
+        expect(() => indigoGrad.add(aColor), throwsUnsupportedError);
+      });
+      test('growable list', () {
+        final growable = const GradOf.indigo(growable: true).colors;
+        final prevLength = growable.length;
+        growable.add(aColor);
+        expect(growable.length, prevLength + 1);
+      });
+    });
+    group('indigo accent:', () {
+      final indigoAccentGrad = const GradOf.indigoAccent().colors;
+      test('gradient', () {
+        expect(indigoAccentGrad, List.of(const IndigoAccents().colors));
+      });
+      test('fixed-length list', () {
+        expect(() => indigoAccentGrad.add(aColor), throwsUnsupportedError);
+      });
+      test('growable list', () {
+        final growable = const GradOf.indigoAccent(growable: true).colors;
+        final prevLength = growable.length;
+        growable.add(aColor);
+        expect(growable.length, prevLength + 1);
+      });
+    });
     group('green:', () {
       final greenGrad = const GradOf.green().colors;
       test('gradient', () {