Don't throw assertion when dart:lib cannot be found.

compiler/java/com/google/dart/compiler/SystemLibraryManager.java

 // Copyright (c) 2011, 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.
 
 package com.google.dart.compiler;
 
 import java.io.File;
 import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URI;
@@ skipping 112 matching lines @@
       if (host == null) {
         String spec = uri.getSchemeSpecificPart();
         String replacement = expansionMap.get(spec);
         if (replacement != null) {
           try {
             uri = new URI(DART_SCHEME + ":" + replacement);
           } catch (URISyntaxException e) {
             throw new AssertionError();
           }
         } else {
-          throw new RuntimeException("Don't know how to expand dart URI: " + uri);
+          return null;
         }
       }
     }
     return uri;
   }
 
   /**
    * Answer <code>true</code> if the specified URI has a "dart" scheme
    */
   public static boolean isDartUri(URI uri) {
@@ skipping 161 matching lines @@
         if (library != null) {
           defaultLibraries.add(library);
           baseFiles[path.ordinal()] = library.getFile();
         }
       }
     }
 
     return defaultLibraries.toArray(new SystemLibrary[defaultLibraries.size()]);
   }
 }