Support deferred libraries in pub build/serve.

sdk/lib/_internal/pub/test/dart2js/outputs_deferred_libraries_test.dart

+// Copyright (c) 2014, 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:scheduled_test/scheduled_test.dart';
+
+import '../descriptor.dart' as d;
+import '../test_pub.dart';
+
+const MAIN = """
+import 'dart:async';
+
+@lazyA import 'a.dart' as a;
+@lazyB import 'b.dart' as b;
+
+const lazyA = const DeferredLibrary('a', uri: 'a.js');
+const lazyB = const DeferredLibrary('b', uri: 'b.js');
+
+void main() {
+  Future.wait([lazyA.load(), lazyB.load()]).then((_) {
+    a.fn();
+    b.fn();
+  });
+}
+""";
+
+const A = """
+library a;
+
+fn() => print("a");
+""";
+
+const B = """
+library b;
+
+fn() => print("b");
+""";
+
+main() {
+  initConfig();
+  integration("compiles deferred libraries to separate outputs", () {
+    // Dart2js can take a long time to compile dart code, so we increase the
+    // timeout to cope with that.
+    currentSchedule.timeout *= 3;
+
+    d.dir(appPath, [
+      d.appPubspec(),
+      d.dir('web', [
+        d.file('main.dart', MAIN),
+        d.file('a.dart', A),
+        d.file('b.dart', B)
+      ])
+    ]).create();
+
+    schedulePub(args: ["build"],
+        output: new RegExp(r'Built 4 files to "build".'));
+
+    d.dir(appPath, [
+      d.dir('build', [
+        d.dir('web', [
+          d.matcherFile('main.dart.js', isNot(isEmpty)),
+          d.matcherFile('main.dart.precompiled.js', isNot(isEmpty)),
+          d.matcherFile('main.dart.js_a.part.js', isNot(isEmpty)),
+          d.matcherFile('main.dart.js_b.part.js', isNot(isEmpty)),
+        ])
+      ])
+    ]).validate();
+  });
+}