Added field isStandalone to GeneratorBuilder to allow generation of a… (#86)

* Added field isStandalone to GeneratorBuilder to allow generation of a file that is not part of the source file.

* Added tests for isStandalone builder option.

* Throw error when 2+ generators used w/ standalone

- When both more than one generator and isStandalone: true passed to
  GeneratorBuilder, an ArgumentError is thrown.
- Wrote test to verify that this happens.

* Actually have to include isStandalone in ArgumentError check.

* Added test for non-standalone, multiple generators

* Ran dartfmt.

Author: samschendel

lib/src/builder.dart

@@ -14,10 +14,16 @@ import 'utils.dart';
 class GeneratorBuilder extends Builder {
   final List<Generator> generators;
   final String generatedExtension;
+  final bool isStandalone;
 
-  GeneratorBuilder(this.generators, {this.generatedExtension: '.g.dart'}) {
+  GeneratorBuilder(this.generators,
+      {this.generatedExtension: '.g.dart', this.isStandalone: false}) {
     // TODO: validate that generatedExtension starts with a `.'
     //       not null, empty, etc
+    if (this.isStandalone && this.generators.length > 1) {
+      throw new ArgumentError(
+          'Only one generator can be used to generate a standalone file.');
+    }
   }
 
   @override
@@ -49,8 +55,10 @@ class GeneratorBuilder extends Builder {
 
     var contentBuffer = new StringBuffer();
 
-    contentBuffer.writeln('part of ${library.name};');
-    contentBuffer.writeln();
+    if (!isStandalone) {
+      contentBuffer.writeln('part of ${library.name};');
+      contentBuffer.writeln();
+    }
 
     for (GeneratedOutput output in generatedOutputs) {
       contentBuffer.writeln('');

test/builder_test.dart

@@ -26,6 +26,41 @@ void main() {
         {'$pkgName|lib/test_lib.g.dart': contains('not valid code!'),});
   });
 
+  test('Generate standalone output file', () async {
+    var srcs = _createPackageStub(pkgName);
+    var phaseGroup = new PhaseGroup.singleAction(
+        new GeneratorBuilder([const CommentGenerator()], isStandalone: true),
+        new InputSet(pkgName, ['lib/test_lib.dart']));
+    await testPhases(phaseGroup, pkgName, srcs,
+        {'$pkgName|lib/test_lib.g.dart': _testGenStandaloneContent,});
+  });
+
+  test('Generate explicitly non-standalone output file', () async {
+    var srcs = _createPackageStub(pkgName);
+    var phaseGroup = new PhaseGroup.singleAction(
+        new GeneratorBuilder([const CommentGenerator()], isStandalone: false),
+        new InputSet(pkgName, ['lib/test_lib.dart']));
+    await testPhases(phaseGroup, pkgName, srcs,
+        {'$pkgName|lib/test_lib.g.dart': _testGenPartContent,});
+  });
+
+  test('Expect error when multiple generators used on a standalone builder',
+      () async {
+    expect(
+        () => new GeneratorBuilder(
+            [const CommentGenerator(), const _NoOpGenerator()],
+            isStandalone: true),
+        throwsA(new isInstanceOf<ArgumentError>()));
+  });
+
+  test('Expect no error when multiple generators used on nonstandalone builder',
+      () async {
+    expect(
+        () => new GeneratorBuilder(
+            [const CommentGenerator(), const _NoOpGenerator()]),
+        returnsNormally);
+  });
+
   test(
       'Simple Generator test for library',
       () => _generateTest(
@@ -151,6 +186,23 @@ part of test_lib;
 // Code for "test_lib"
 ''';
 
+const _testGenStandaloneContent = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
+
+// **************************************************************************
+// Generator: CommentGenerator
+// Target: class Person
+// **************************************************************************
+
+// Code for "class Person"
+
+// **************************************************************************
+// Generator: CommentGenerator
+// Target: class Customer
+// **************************************************************************
+
+// Code for "class Customer"
+''';
+
 const _testGenPartContentForClassesAndLibrary =
     r'''// GENERATED CODE - DO NOT MODIFY BY HAND