Fixed crash when compiling without linking.

lib/binary/ast_to_binary.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.
 library kernel.ast_to_binary;
 
 import '../ast.dart';
 import '../import_table.dart';
 import 'tag.dart';
 import 'dart:convert';
 import 'dart:typed_data';
@@ skipping 74 matching lines @@
     for (var entry in indexer.entries) {
       writeStringTableEntry(entry.value);
     }
   }
 
   void writeStringReference(String string) {
     writeUInt30(_stringIndexer[string]);
   }
 
   void writeUriReference(String string) {
-    writeUInt30(_sourceUriIndexer[string]);
+    int index = _sourceUriIndexer[string];
+    if (index == null) {
+      // Assume file was loaded without linking. Bail out to empty string.
+      index = _sourceUriIndexer[""];
+    }
+    writeUInt30(index);
   }
 
   void writeList(List items, writeItem(x)) {
     writeUInt30(items.length);
     items.forEach(writeItem);
   }
 
   void writeNodeList(List<Node> nodes) {
     writeList(nodes, writeNode);
   }
@@ skipping 1138 matching lines @@
   void flush() {
     _sink.add(_buffer.sublist(0, length));
     _buffer = new Uint8List(SIZE);
     length = 0;
   }
 
   void flushAndDestroy() {
     _sink.add(_buffer.sublist(0, length));
   }
 }