Make dart2js not consider dart:io there if the library is unsupported.

tests/language/conditional_import_string_test.dart

+// Copyright (c) 2016, 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";
+
+// All three libraries have an HttpRequest class.
+import "conditional_import_string_test.dart"
+    if (dart.library.io == "true") "dart:io"
+    if (dart.library.html == "true") "dart:html"
+    deferred as d show HttpRequest;
+
+class HttpRequest {}
+
+void main() {
+  asyncStart();
+  var io = const String.fromEnvironment("dart.library.io");
+  var html = const String.fromEnvironment("dart.library.html");
+  () async {
+    // Shouldn't fail. Shouldn't time out.
+    await d.loadLibrary().timeout(const Duration(seconds: 5));
+    if (io == "true") {
+      print("io");
+      Expect.throws(() => new d.HttpRequest());  // Class is abstract in dart:io
+    } else if (html == "true") {
+      print("html");
+      dynamic r = new d.HttpRequest();  // Shouldn't throw
+      var o = r.open;  // Shouldn't fail, the open method is there.
+    } else {
+      print("none");
+      dynamic r = new d.HttpRequest();
+      Expect.isTrue(r is HttpRequest);
+    }
+    asyncEnd();
+  }();
+}
+