Temporarily allow to import dart:io in client apps.

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 204 matching lines @@
               resolutionInput, sourceFile.slowText());
         });
       });
     }
     if (resolvedUriTranslator.isNotSet) {
       future = future.then((_) {
         return platform_configuration
             .load(options.platformConfigUri, provider)
             .then((Map<String, Uri> mapping) {
           resolvedUriTranslator.resolvedUriTranslator =
-              new ResolvedUriTranslator(mapping, reporter);
+              new ResolvedUriTranslator(
+                  mapping, reporter, options.platformConfigUri);
         });
       });
     }
     // TODO(johnniwinther): This does not apply anymore.
     // The incremental compiler sets up the sdk before run.
     // Therefore this will be called a second time.
     return future;
   }
 
   Future<bool> run(Uri uri) {
@@ skipping 149 matching lines @@
     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";
     }
+
+    // Note: we return null on `dart:io` here, even if we allow users to
+    // unconditionally import it.
+    //
+    // In the past it was invalid to import `dart:io` for client apps. We just
+    // made it valid to import it as a stopgap measure to support packages like
+    // `http`. This is temporary until we support config-imports in the
+    // language.
+    //
+    // Because it is meant to be temporary and because the returned `dart:io`
+    // implementation will throw on most APIs, we still preserve that
+    // when compiling client apps the `dart:io` library is technically not
+    // supported, and so `const bool.fromEnvironment(dart.library.io)` is false.
     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.';