Add option for outputting a deferred loading mapping.

tests/compiler/dart2js/deferred_load_mapping_test.dart

+// Copyright (c) 2015, 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.
+
+import 'package:expect/expect.dart';
+import "package:async_helper/async_helper.dart";
+import 'memory_source_file_helper.dart';
+import "memory_compiler.dart";
+
+void main() {
+  var collector = new OutputCollector();
+  Compiler compiler = compilerFor(MEMORY_SOURCE_FILES,
+                                  options: ['--deferred-map=deferred_map.json'],
+                                  outputProvider: collector);
+  asyncTest(() {
+    return compiler.run(Uri.parse('memory:main.dart')).then((success) {
+      // Ensure a mapping file is output.
+      Expect.isNotNull(
+          collector.getOutput("deferred_map.json", "deferred_map"));
+
+      Map mapping = compiler.deferredLoadTask.deferredMap();
+      // Test structure of mapping.
+      Expect.isTrue(mapping["main.dart"]["lib1"].length == 2);
+      Expect.isTrue(mapping["main.dart"]["lib2"].length == 2);
+      Expect.isTrue(mapping["main.dart"]["convert"].length == 1);
+      Expect.isTrue(mapping["memory:lib1.dart"]["lib4_1"].length == 1);
+      Expect.isTrue(mapping["memory:lib2.dart"]["lib4_2"].length == 1);
+    });
+  });
+}
+
+const Map MEMORY_SOURCE_FILES = const {
+  "main.dart":"""
+import 'dart:convert' deferred as convert;
+import 'lib1.dart' deferred as lib1;
+import 'lib2.dart' deferred as lib2;
+
+void main() {
+  lib1.loadLibrary().then((_) {
+        lib1.foo1();
+        new lib1.C();
+    lib2.loadLibrary().then((_) {
+        lib2.foo2();
+    });
+  });
+  convert.loadLibrary().then((_) {
+    new convert.JsonCodec();
+  });
+}
+""",
+  "lib1.dart":"""
+library lib1;
+import "dart:async";
+import "dart:html";
+
+import "lib3.dart" as l3;
+import "lib4.dart" deferred as lib4_1;
+
+class C {}
+
+foo1() {
+  new InputElement();
+  lib4_1.loadLibrary().then((_) {
+    lib4_1.bar1();
+  });
+  return () {return 1 + l3.foo3();} ();
+}
+""",
+  "lib2.dart":"""
+library lib2;
+import "dart:async";
+import "lib3.dart" as l3;
+import "lib4.dart" deferred as lib4_2;
+
+foo2() {
+  lib4_2.loadLibrary().then((_) {
+    lib4_2.bar2();
+  });
+  return () {return 2+l3.foo3();} ();
+}
+""",
+  "lib3.dart":"""
+library lib3;
+
+foo3() {
+  return () {return 3;} ();
+}
+""",
+  "lib4.dart":"""
+library lib4;
+
+bar1() {
+  return "hello";
+}
+
+bar2() {
+  return 2;
+}
+""",
+};