Add MultiSourceInformationStrategy

tests/compiler/dart2js/output_collector.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // Output provider that collects the output in string buffers.
 
 library output_collector;
 
 import 'dart:async';
 import 'package:compiler/compiler_new.dart';
@@ skipping 26 matching lines @@
   void close() {
     sinks.forEach((OutputSink sink) => sink.close());
   }
 }
 
 class OutputCollector implements CompilerOutput {
   Map<OutputType, Map<String, BufferedOutputSink>> outputMap = {};
 
   String getOutput(String name, OutputType type) {
     Map<String, BufferedOutputSink> sinkMap = outputMap[type];
-    if (sinkMap == null) return null;
+    if (sinkMap == null) {
+      print("No output available for $type.");
+      return null;
+    }
     BufferedOutputSink sink = sinkMap[name];
-    return sink != null ? sink.text : null;
+    if (sink == null) {
+      print("Output '$name' not found for $type. Available: ${sinkMap.keys}");
+      return null;
+    } else {
+      return sink.text;
+    }
   }
 
   /// `true` if any output has been collected.
   bool get hasOutput => !outputMap.isEmpty;
 
   /// `true` if any output other than main output has been collected.
   bool get hasExtraOutput {
     for (OutputType type in outputMap.keys) {
       for (String name in outputMap[type].keys) {
         if (name != '') return true;
       }
     }
     return false;
   }
 
   @override
   OutputSink createOutputSink(String name, String extension, OutputType type) {
     Map<String, BufferedOutputSink> sinkMap =
         outputMap.putIfAbsent(type, () => {});
     return sinkMap.putIfAbsent(name, () => new BufferedOutputSink());
   }
 }