When registering deferred constant, use canonical output unit.

pkg/compiler/lib/src/dump_info.dart

Index: pkg/compiler/lib/src/dump_info.dart
diff --git a/pkg/compiler/lib/src/dump_info.dart b/pkg/compiler/lib/src/dump_info.dart
index 6803b0532857c76525207e88b9159770bd69d9e7..2afb84928c48f2e7cc450e7f4eb100d1c3ddc9ba 100644
--- a/pkg/compiler/lib/src/dump_info.dart
+++ b/pkg/compiler/lib/src/dump_info.dart
@@ -320,6 +320,7 @@ class ElementInfoCollector extends BaseElementVisitor<Info, dynamic> {
       // 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