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

pkg/compiler/lib/src/apiimpl.dart

 // Copyright (c) 2012, 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.
 
 library leg_apiimpl;
 
 import 'dart:async';
 
 import 'package:package_config/packages.dart';
 import 'package:package_config/packages_file.dart' as pkgs;
@@ skipping 363 matching lines @@
 
     var result = definitions[name];
     if (result != null || definitions.containsKey(name)) return result;
     if (!name.startsWith(_dartLibraryEnvironmentPrefix)) return null;
 
     String libraryName = name.substring(_dartLibraryEnvironmentPrefix.length);
 
     // Private libraries are not exposed to the users.
     if (libraryName.startsWith("_")) return null;
 
-    if (compiler.resolvedUriTranslator.sdkLibraries.containsKey(libraryName)) {
+    Uri libraryUri =
+        compiler.resolvedUriTranslator.sdkLibraries[libraryName];
+    if (libraryUri != null && libraryUri.scheme != "unsupported") {
       // Dart2js always "supports" importing 'dart:mirrors' but will abort
       // the compilation at a later point if the backend doesn't support
       // mirrors. In this case 'mirrors' should not be in the environment.
       if (libraryName == 'mirrors') {
         return compiler.backend.supportsReflection ? "true" : null;
       }
       return "true";
     }
     return null;
   }
 }
 
 /// For every 'dart:' library, a corresponding environment variable is set
 /// to "true". The environment variable's name is the concatenation of
 /// this prefix and the name (without the 'dart:'.
 ///
 /// For example 'dart:html' has the environment variable 'dart.library.html' set
 /// to "true".
 const String _dartLibraryEnvironmentPrefix = 'dart.library.';