misc(_Builder): handle edge case where generator toString is multiline

- also moved "rendering" of the `Generator:` prefix to GeneratedOutput
- Sets up changes in future commits

Author: kevmoo

source_gen/lib/src/builder.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:build/build.dart';
@@ -106,14 +107,15 @@ class _Builder extends Builder {
       contentBuffer.writeln('part of $name;');
     }
 
-    for (var output in generatedOutputs) {
+    for (var item in generatedOutputs) {
       contentBuffer
         ..writeln()
         ..writeln(_headerLine)
-        ..writeln('// Generator: ${output.generator}')
+        ..writeAll(
+            LineSplitter.split(item.toString()).map((line) => '// $line\n'))
         ..writeln(_headerLine)
         ..writeln()
-        ..write(output.output);
+        ..write(item.output);
     }
 
     var genPartContent = contentBuffer.toString();

source_gen/lib/src/generated_output.dart

@@ -21,6 +21,9 @@ class GeneratedOutput {
 
   GeneratedOutput.fromError(this.generator, this.error, this.stackTrace)
       : this.output = _outputFromError(error);
+
+  @override
+  String toString() => 'Generator: $generator';
 }
 
 String _outputFromError(Object error) {