Temporarily allow to import dart:io in client apps.

pkg/compiler/lib/src/resolved_uri_translator.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 'common.dart';
 import 'elements/elements.dart' show LibraryElement;
 import 'util/emptyset.dart';
 
 /// API used by the library loader to translate internal SDK URIs into file
 /// system readable URIs.
 abstract class ResolvedUriTranslator {
   factory ResolvedUriTranslator(
-          Map<String, Uri> sdkLibraries, DiagnosticReporter reporter) =
-      _ResolvedUriTranslator;
+      Map<String, Uri> sdkLibraries,
+      DiagnosticReporter reporter,
+      Uri platformConfigUri) = _ResolvedUriTranslator;
 
   /// The set of platform libraries reported as unsupported.
   ///
   /// For instance when importing 'dart:io' without '--categories=Server'.
   Set<Uri> get disallowedLibraryUris;
 
   /// Whether or not a mockable library has been translated.
   bool get mockableLibraryUsed;
 
   /// A mapping from dart: library names to their location.
@@ skipping 44 matching lines @@
   @override
   bool get mockableLibraryUsed => resolvedUriTranslator.mockableLibraryUsed;
 
   @override
   Map<String, Uri> get sdkLibraries => resolvedUriTranslator.sdkLibraries;
 }
 
 class _ResolvedUriTranslator implements ResolvedUriTranslator {
   final Map<String, Uri> _sdkLibraries;
   final DiagnosticReporter _reporter;
+  final Uri _platformConfigUri;
 
   Set<Uri> disallowedLibraryUris = new Set<Uri>();
   bool mockableLibraryUsed = false;
 
-  _ResolvedUriTranslator(this._sdkLibraries, this._reporter);
+  _ResolvedUriTranslator(
+      this._sdkLibraries, this._reporter, this._platformConfigUri);
 
   Map<String, Uri> get sdkLibraries => _sdkLibraries;
 
   @override
   Uri translate(LibraryElement importingLibrary, Uri uri, Spannable spannable) {
     if (uri.scheme == 'dart') {
       return translateDartUri(importingLibrary, uri, spannable);
     }
     return uri;
   }
@@ skipping 35 matching lines @@
         } else {
           _reporter.reportErrorMessage(spannable, MessageKind.INTERNAL_LIBRARY,
               {'resolvedUri': resolvedUri});
           registerDisallowedLibraryUse(resolvedUri);
         }
         return null;
       }
     }
 
     if (location.scheme == "unsupported") {
-      _reporter.reportErrorMessage(spannable, MessageKind.LIBRARY_NOT_SUPPORTED,
-          {'resolvedUri': resolvedUri});
-      registerDisallowedLibraryUse(resolvedUri);
+      if (location.path == "") {
+        _reporter.reportErrorMessage(spannable,
+            MessageKind.LIBRARY_NOT_SUPPORTED, {'resolvedUri': resolvedUri});
+        registerDisallowedLibraryUse(resolvedUri);
+      } else {
+        // If the specification includes a path, we treat it as "partially"
+        // unsupported: it is allowed to be imported unconditionally, but we
+        // will not expose it as being supported in the const variable
+        // `dart.library.name`.
+        //
+        // This is a stopgap measure to support packages like `http` that need
+        // to import `dart:io` conditionally. Once config-imports are supported
+        // in the language, we can make it an error again to import it
+        // unconditionally.
+        //
+        // The plaform configuration files contain a URI of the form
+        // `unsupported:path/to/library.dart` to indicate this partially
+        // supported mode. We resolve the path with respect to the configuration
+        // file.
+        return _platformConfigUri.resolve(location.path);
+      }
       return null;
     }
 
     if (resolvedUri.path == 'html' || resolvedUri.path == 'io') {
       // TODO(ahe): Get rid of mockableLibraryUsed when test.dart
       // supports this use case better.
       mockableLibraryUsed = true;
     }
     return location;
   }
 
   void registerDisallowedLibraryUse(Uri uri) {
     disallowedLibraryUris.add(uri);
   }
 
   Uri lookupLibraryUri(String libraryName) {
     return _sdkLibraries[libraryName];
   }
 }