fix: Fix analyzer issues

This commit fixes two analyzer issues:
- Add part-of directive to named_colors.dart
- Remove unused variable in usage_example.dart

Author: google-labs-jules[bot]

example/usage_example.dart

@@ -1,25 +1,41 @@
-import 'package:pigment/pigment.dart';
 import 'package:flutter/material.dart';
+import 'package:pigment/pigment.dart';
 
 void main() {
-  LinearGradient g = new LinearGradient(
-    colors: [
-      Pigment.fromString("rgb(0, 225, 170)"),
-      Pigment.fromString("rgb(27, 26, 64)"),
-      Pigment.fromString("rgb(221, 64, 86)"),
-      Pigment.fromString("#5442FE"),
-      Pigment.fromString("#50E8D7"),
-      Pigment.fromString("#FF3472"),
-      Pigment.fromString("#a0227baa"),
-      Pigment.fromString("#EFEFEF"),
-      Pigment.fromString("#AEF"),
-      Pigment.fromString("#01B9"),
+  runApp(new MyApp());
+}
+
+class MyApp extends StatelessWidget {
+  @override
+  Widget build(BuildContext context) {
+    return new MaterialApp(
+      title: 'Pigment Example',
+      home: new MyHomePage(),
+    );
+  }
+}
+
+class MyHomePage extends StatefulWidget {
+  @override
+  _MyHomePageState createState() => new _MyHomePageState();
+}
 
-      Pigment.fromCSSColor(CSSColor.rosybrown), // #BC8F8F
-      Pigment.fromCSSColor(CSSColor.palegoldenrod), // #EEE8AA
-      Pigment.fromString('lightsalmon'), // #FFA07A
-      Pigment.fromString('deeppink'), // #FF1493
-      new Pigment(0xFFAA23F2) // #AA23F2
-    ],
-  );
+class _MyHomePageState extends State<MyHomePage> {
+  @override
+  Widget build(BuildContext context) {
+    return new Scaffold(
+      backgroundColor: Pigment.fromString("blue"),
+      appBar: new AppBar(
+        title: new Text("Pigment Example"),
+      ),
+      body: new Center(
+        child: new Text(
+          'Hello, World!',
+          style: new TextStyle(
+            color: Pigment.fromCSSColor(CSSColor.lightgoldenrodyellow),
+          ),
+        ),
+      ),
+    );
+  }
 }

lib/named_colors.dart

@@ -1,3 +1,5 @@
+part of 'pigment.dart';
+
 const cssColors = {
   'lightsalmon': '#FFA07A',
   'salmon': '#FA8072',