Add option for outputting a deferred loading mapping.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 673fa5ba79aac85fd2d5d9284b569acc53350f70..d15fe60eaca5813365166bfdd4483068ae33d533 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -668,6 +668,7 @@ abstract class Compiler implements DiagnosticListener {
   final bool trustPrimitives;
   final bool enableConcreteTypeInference;
   final bool disableTypeInferenceFlag;
+  final Uri deferredMapUri;
   final bool dumpInfo;
   final bool useContentSecurityPolicy;
   final bool enableExperimentalMirrors;
@@ -984,6 +985,7 @@ abstract class Compiler implements DiagnosticListener {
             this.outputUri: null,
             this.buildId: UNDETERMINED_BUILD_ID,
             this.terseDiagnostics: false,
+            this.deferredMapUri: null,
             this.dumpInfo: false,
             this.showPackageWarnings: false,
             this.useContentSecurityPolicy: false,
@@ -1532,7 +1534,7 @@ abstract class Compiler implements DiagnosticListener {
     // compile-time constants that are metadata.  This means adding
     // something to the resolution queue.  So we cannot wait with
     // this until after the resolution queue is processed.
-    deferredLoadTask.ensureMetadataResolved(this);
+    deferredLoadTask.beforeResolution(this);
 
     phase = PHASE_RESOLVING;
     if (analyzeAll) {