dart2js: add support for configuration-specific imports.

pkg/compiler/lib/src/tree/unparser.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.
 
 part of tree;
 
 String unparse(Node node, {minify: true}) {
   Unparser unparser = new Unparser(minify: minify);
   unparser.unparse(node);
   return unparser.result;
@@ skipping 764 matching lines @@
     write(node.endToken.value);
   }
 
   visitLibraryName(LibraryName node) {
     addToken(node.libraryKeyword);
     node.visitChildren(this);
     write(node.getEndToken().value);
     newline();
   }
 
+  visitConditionalUri(ConditionalUri node) {
+    write(node.ifToken.value);
+    space();
+    write('(');
+    visit(node.key);
+    if (node.value != null) {
+      space();
+      write("==");
+      space();
+      visit(node.value);
+    }
+    write(")");
+    space();
+    visit(node.uri);
+  }
+
+  visitDottedName(DottedName node) {
+    unparseNodeListOfIdentifiers(node.identifiers);
+  }
+
   visitImport(Import node) {
     addToken(node.importKeyword);
     visit(node.uri);
+    if (node.hasConditionalUris) {
+      write(' ');
+      visitNodeList(node.conditionalUris);
+    }
     if (node.isDeferred) {
       write(' deferred');
     }
     if (node.prefix != null) {
       write(' as ');
       visit(node.prefix);
     }
     if (node.combinators != null) {
       write(' ');
       visit(node.combinators);
     }
     write(node.getEndToken().value);
     newline();
   }
 
   visitExport(Export node) {
     addToken(node.exportKeyword);
     visit(node.uri);
+    if (node.hasConditionalUris) {
+      write(' ');
+      visitNodeList(node.conditionalUris);
+    }
     if (node.combinators != null) {
       write(' ');
       visit(node.combinators);
     }
     write(node.getEndToken().value);
     newline();
   }
 
   visitPart(Part node) {
     addToken(node.partKeyword);
@@ skipping 51 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }