Add interfaces for a new compiler API.

pkg/docgen/lib/src/generator.dart

Index: pkg/docgen/lib/src/generator.dart
diff --git a/pkg/docgen/lib/src/generator.dart b/pkg/docgen/lib/src/generator.dart
index 7df02600fd71def4975451c2698f20f7711a0342..18ed895ec673f1276e862ca580e03f3dffb59538 100644
--- a/pkg/docgen/lib/src/generator.dart
+++ b/pkg/docgen/lib/src/generator.dart
@@ -268,10 +268,10 @@ void _ensureOutputDirectory(String outputDirectory) {
 Future<MirrorSystem> analyzeLibraries(List<Uri> libraries, String
     libraryRoot, {String packageRoot}) {
   SourceFileProvider provider = new CompilerSourceFileProvider();
-  api.DiagnosticHandler diagnosticHandler = (new FormattingDiagnosticHandler(
+  api.DiagnosticHandler diagnosticHandler = new FormattingDiagnosticHandler(
       provider)
       ..showHints = false
-      ..showWarnings = false).diagnosticHandler;
+      ..showWarnings = false;
   Uri libraryUri = new Uri.file(appendSlash(libraryRoot));
   Uri packageUri = null;
   if (packageRoot == null) {