Example: add GeneratorForAnnotation example

More progress towards #322

example/build.yaml

@@ -28,3 +28,11 @@ builders:
     auto_apply: dependents
     build_to: cache
     applies_builders: ["source_gen|combining_builder"]
+
+  property_multiply:
+    import: "package:source_gen_example/builder.dart"
+    builder_factories: ["multiplyBuilder"]
+    build_extensions: {".dart": ["multiply.g.part"]}
+    auto_apply: dependents
+    build_to: cache
+    applies_builders: ["source_gen|combining_builder"]

example/lib/annotations.dart

@@ -0,0 +1,9 @@
+// Copyright (c) 2018, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+class Multiplier {
+  final num value;
+
+  const Multiplier(this.value);
+}

example/lib/builder.dart

@@ -16,6 +16,7 @@ import 'package:build/build.dart';
 import 'package:source_gen/source_gen.dart';
 
 import 'src/member_count_library_generator.dart';
+import 'src/multiplier_generator.dart';
 import 'src/property_product_generator.dart';
 import 'src/property_sum_generator.dart';
 
@@ -28,3 +29,6 @@ Builder productBuilder(BuilderOptions options) =>
 
 Builder sumBuilder(BuilderOptions options) =>
     SharedPartBuilder([PropertySumGenerator()], 'sum');
+
+Builder multiplyBuilder(BuilderOptions options) =>
+    SharedPartBuilder([MultiplierGenerator()], 'multiply');

example/lib/src/multiplier_generator.dart

@@ -0,0 +1,19 @@
+// Copyright (c) 2018, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:analyzer/dart/element/element.dart';
+import 'package:build/build.dart';
+import 'package:source_gen/source_gen.dart';
+
+import '../annotations.dart';
+
+class MultiplierGenerator extends GeneratorForAnnotation<Multiplier> {
+  @override
+  String generateForAnnotatedElement(
+      Element element, ConstantReader annotation, BuildStep buildStep) {
+    final numValue = annotation.read('value').literalValue as num;
+
+    return 'num ${element.name}Multiplied() => ${element.name} * $numValue;';
+  }
+}

example_usage/build.yaml

@@ -8,3 +8,9 @@ targets:
         # Only run this builder on the specified input.
         generate_for:
           - lib/library_source.dart
+      source_gen_example|property_sum:
+        generate_for:
+          - lib/*.dart
+      source_gen_example|property_product:
+        generate_for:
+          - lib/*.dart

example_usage/lib/library_source.dart

@@ -4,7 +4,11 @@
 
 import 'dart:math';
 
+import 'package:source_gen_example/annotations.dart';
+
 part 'library_source.g.dart';
 
+@Multiplier(2)
 final answer = 42;
+
 final tau = pi * 2;