When registering deferred constant, use canonical output unit.

pkg/compiler/lib/src/dump_info.dart

 // Copyright (c) 2013, 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.
 
 library dump_info;
 
 import 'dart:convert'
     show ChunkedConversionSink, JsonEncoder, StringConversionSink;
 
 import 'package:dart2js_info/info.dart';
@@ skipping 302 matching lines @@
     result.functions.add(info);
     return info;
   }
 
   OutputUnitInfo _infoFromOutputUnit(OutputUnit outputUnit) {
     return _outputToInfo.putIfAbsent(outputUnit, () {
       // Dump-info currently only works with the full emitter. If another
       // emitter is used it will fail here.
       JavaScriptBackend backend = compiler.backend;
       full.Emitter emitter = backend.emitter.emitter;
+      assert(outputUnit.name != null || outputUnit.isMainOutput);
       OutputUnitInfo info = new OutputUnitInfo(
           outputUnit.name, emitter.outputBuffers[outputUnit].length);
       info.imports.addAll(outputUnit.imports
           .map((d) => compiler.deferredLoadTask.importDeferName[d]));
       result.outputUnits.add(info);
       return info;
     });
   }
 
   OutputUnitInfo _unitInfoForElement(Element element) {
@@ skipping 252 matching lines @@
 
     ChunkedConversionSink<Object> sink = encoder.startChunkedConversion(
         new StringConversionSink.fromStringSink(buffer));
     sink.add(new AllInfoJsonCodec().encode(result));
     compiler.reporter.reportInfo(NO_LOCATION_SPANNABLE, MessageKind.GENERIC, {
       'text': "View the dumped .info.json file at "
           "https://dart-lang.github.io/dump-info-visualizer"
     });
   }
 }