misc(Generator): simplify how the source generator is printed in source

If `toString` on a generator ends in `Generator` output this:

// *********************************************************************
// CommentGenerator
// *********************************************************************

Instead of this:

// *********************************************************************
// Generator: CommentGenerator
// *********************************************************************

Author: kevmoo

source_gen/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 0.8.2
+
+* Simplification to the output of generator names in header sections.
+
 ## 0.8.1+3
 
 * More redundant new lines elimination.

source_gen/lib/src/generated_output.dart

@@ -23,7 +23,13 @@ class GeneratedOutput {
       : this.output = _outputFromError(error);
 
   @override
-  String toString() => 'Generator: $generator';
+  String toString() {
+    var output = generator.toString();
+    if (output.endsWith('Generator')) {
+      return output;
+    }
+    return 'Generator: $output';
+  }
 }
 
 String _outputFromError(Object error) {

source_gen/test/builder_test.dart

@@ -273,7 +273,7 @@ const _testGenPartContent = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
 part of test_lib;
 
 // **************************************************************************
-// Generator: CommentGenerator
+// CommentGenerator
 // **************************************************************************
 
 // Code for "class Person"
@@ -286,7 +286,7 @@ const _testGenPartContentForLibrary =
 part of test_lib;
 
 // **************************************************************************
-// Generator: CommentGenerator
+// CommentGenerator
 // **************************************************************************
 
 // Code for "test_lib"
@@ -295,7 +295,7 @@ part of test_lib;
 const _testGenStandaloneContent = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
 
 // **************************************************************************
-// Generator: CommentGenerator
+// CommentGenerator
 // **************************************************************************
 
 // Code for "class Person"
@@ -308,7 +308,7 @@ const _testGenPartContentForClassesAndLibrary =
 part of test_lib;
 
 // **************************************************************************
-// Generator: CommentGenerator
+// CommentGenerator
 // **************************************************************************
 
 // Code for "test_lib"
@@ -321,7 +321,7 @@ const _testGenPartContentError = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
 part of test_lib;
 
 // **************************************************************************
-// Generator: CommentGenerator
+// CommentGenerator
 // **************************************************************************
 
 // Error: Don't use classes with the word 'Error' in the name
@@ -336,7 +336,7 @@ const _testGenNoLibrary = r'''// GENERATED CODE - DO NOT MODIFY BY HAND
 part of 'test_lib.dart';
 
 // **************************************************************************
-// Generator: CommentGenerator
+// CommentGenerator
 // **************************************************************************
 
 // Code for "class A"