dart2js: fix try/poi after refactor

pkg/dart2js_incremental/lib/caching_compiler.dart

Index: pkg/dart2js_incremental/lib/caching_compiler.dart
diff --git a/pkg/dart2js_incremental/lib/caching_compiler.dart b/pkg/dart2js_incremental/lib/caching_compiler.dart
index eceea65aa8ea82a9afd2e7011388f899f41a5e1a..2bc2047dcbfdc0ddb44f01782ba2a15d913d184e 100644
--- a/pkg/dart2js_incremental/lib/caching_compiler.dart
+++ b/pkg/dart2js_incremental/lib/caching_compiler.dart
@@ -185,10 +185,10 @@ class StringEventSink implements EventSink<String> {
 }
 
 /// Output provider which collect output in [output].
-class OutputProvider {
+class OutputProvider implements CompilerOutput {
   final Map<String, String> output = new Map<String, String>();
 
-  EventSink<String> call(String name, String extension) {
+  EventSink<String> createEventSink(String name, String extension) {
     return new StringEventSink((String data) {
       output['$name.$extension'] = data;
     });