dart2js: add support for configuration-specific imports.

pkg/compiler/lib/src/library_loader.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 dart2js.library_loader;
 
 import 'dart:async';
 
 import 'common/names.dart' show
     Uris;
@@ skipping 373 matching lines @@
 
     bool importsDartCore = false;
     LinkBuilder<LibraryDependencyElementX> libraryDependencies =
         new LinkBuilder<LibraryDependencyElementX>();
     Uri base = library.entryCompilationUnit.script.readableUri;
 
     return Future.forEach(library.tags, (LibraryTag tag) {
       return compiler.withCurrentElement(library, () {
 
         Uri computeUri(LibraryDependency node) {
-          String tagUriString = node.uri.dartString.slowToString();
+          StringNode uriNode = node.uri;
+          if (node.conditionalUris != null) {
+            for (ConditionalUri conditionalUri in node.conditionalUris) {
+              String key = conditionalUri.key.slowNameString;
+              String value = conditionalUri.value == null
+                  ? "true"
+                  : conditionalUri.value.dartString.slowToString();
+              String actual = compiler.fromEnvironment(key);
+              if (value == actual) {
+                uriNode = conditionalUri.uri;
+                break;
+              }
+            }
+          }
+          String tagUriString = uriNode.dartString.slowToString();
           try {
             return Uri.parse(tagUriString);
           } on FormatException {
             compiler.reportErrorMessage(
                 node.uri,
                 MessageKind.INVALID_URI, {'uri': tagUriString});
             return null;
           }
         }
 
@@ skipping 921 matching lines @@
         }
         suffixes.add(const Link<Uri>().prepend(canonicalUri));
       }
       suffixChainMap[library] = suffixes;
       return;
     }
 
     computeSuffixes(rootLibrary, const Link<Uri>());
   }
 }