Add option for outputting a deferred loading mapping.

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 0372b8534d60c2bfe79bd2d09d8f1a47507fb8f3..05734e94e733a8121902b6028432674833daf004 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -333,6 +333,7 @@ Future compile(List<String> argv) {
     new OptionHandler('--categories=.*', setCategories),
     new OptionHandler('--disable-type-inference', implyCompilation),
     new OptionHandler('--terse', passThrough),
+    new OptionHandler('--deferred-map=.+', implyCompilation),
     new OptionHandler('--dump-info', implyCompilation),
     new OptionHandler('--disallow-unsafe-eval',
                       (_) => hasDisallowUnsafeEval = true),
@@ -598,6 +599,10 @@ be removed in a future version:
     unsupported category, for example, --categories=help.  To enable
     all categories, use --categories=all.
 
+  --deferred-map=<file>
+    Generates a json file with a mapping from each deferred import to a list of
+    the part.js files that will be loaded.
+
   --dump-info
     Generates an out.info.json file with information about the generated code.
     You can inspect the generated file with the viewer at: