Add option for outputting a deferred loading mapping.

pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart

Index: pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
diff --git a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
index 40a747308c37b9f4be70cc4cc5ec6662b73d3d74..ab497435968dc8bb098e2c36998ba02226ff9154 100644
--- a/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
+++ b/pkg/compiler/lib/src/js_emitter/old_emitter/emitter.dart
@@ -1733,6 +1733,10 @@ class OldEmitter implements Emitter {
 
     emitDeferredBoilerPlate(mainBuffer, deferredLoadHashes);
 
+    if (compiler.deferredMapUri != null) {
+      outputDeferredMap();
+    }
+
     // Static field initializations require the classes and compile-time
     // constants to be set up.
     emitStaticNonFinalFieldInitializations(mainBuffer, mainOutputUnit);
@@ -2270,6 +2274,18 @@ function(originalDescriptor, name, holder, isStatic, globalFunctionsAccess) {
         ..close();
   }
 
+  void outputDeferredMap() {
+    Map mapping = new Map();

[floitsch, 2015/01/05 14:10:38]

types.

[sigurdm, 2015/01/05 15:23:08]

I changed it to Map<String, dynamic> If we want the comment to stay, then it
cannot get more precise.

+    // Json does not support comments, so we embed the explanation in the
+    // data.
+    mapping["_comment"] = "This mapping shows which compiled `.js` files are "
+        "needed for a given deferred library import.";
+    mapping.addAll(compiler.deferredLoadTask.deferredMap());
+    compiler.outputProvider(compiler.deferredMapUri.path, 'deferred_map')
+        ..add(const JsonEncoder.withIndent("  ").convert(mapping))
+        ..close();
+  }
+
   void invalidateCaches() {
     if (!compiler.hasIncrementalSupport) return;
     if (cachedElements.isEmpty) return;