This CL is part of a proposal to change the API of dartc's implementation of UrlSource.

compiler/java/com/google/dart/compiler/UrlLibrarySource.java

Index: compiler/java/com/google/dart/compiler/UrlLibrarySource.java
===================================================================
--- compiler/java/com/google/dart/compiler/UrlLibrarySource.java	(revision 4151)
+++ compiler/java/com/google/dart/compiler/UrlLibrarySource.java	(working copy)
@@ -34,7 +34,7 @@
   public DartSource getSourceFor(final String relPath) {
     try {
       // Force the creation of an escaped relative URI to deal with spaces, etc.
-      URI uri = getAbsoluteUri().resolve(new URI(null, null, relPath, null)).normalize();
+      URI uri = getUri().resolve(new URI(null, null, relPath, null)).normalize();
       return new UrlDartSource(uri, relPath, this, systemLibraryManager);
     } catch (URISyntaxException e) {
       throw new AssertionError(e);
@@ -43,6 +43,6 @@
 
   @Override
   public LibrarySource getImportFor(String relPath) {
-    return new UrlLibrarySource(getAbsoluteUri().resolve(relPath).normalize(), systemLibraryManager);
+    return new UrlLibrarySource(getUri().resolve(relPath).normalize(), systemLibraryManager);
   }
 }