Support references to unserialized libraries.

pkg/compiler/lib/src/library_loader.dart

Index: pkg/compiler/lib/src/library_loader.dart
diff --git a/pkg/compiler/lib/src/library_loader.dart b/pkg/compiler/lib/src/library_loader.dart
index efb4893b3940cce97d6f02680b52d4109b603609..9da31d6188cc9103c690665e835a67a71f0c22f1 100644
--- a/pkg/compiler/lib/src/library_loader.dart
+++ b/pkg/compiler/lib/src/library_loader.dart
@@ -131,7 +131,7 @@ typedef Future<Iterable<LibraryElement>> ReuseLibrariesFunction(
  * point to the 'packages' folder.
  *
  */
-abstract class LibraryLoaderTask implements CompilerTask {
+abstract class LibraryLoaderTask implements LibraryProvider, CompilerTask {
   factory LibraryLoaderTask(
       ResolvedUriTranslator uriTranslator,
       ScriptLoader scriptLoader,
@@ -145,9 +145,6 @@ abstract class LibraryLoaderTask implements CompilerTask {
   /// Returns all libraries that have been loaded.
   Iterable<LibraryElement> get libraries;
 
-  /// Looks up the library with the [canonicalUri].
-  LibraryElement lookupLibrary(Uri canonicalUri);
-
   /// Loads the library specified by the [resolvedUri] and returns its
   /// [LibraryElement].
   ///
@@ -176,6 +173,14 @@ abstract class LibraryLoaderTask implements CompilerTask {
   Future<Null> resetLibraries(ReuseLibrariesFunction reuseLibraries);
 }
 
+/// Interface for an entity that provide libraries. For instance from normal
+/// library loading or from deserialization.
+// TODO(johnniwinther): Use this to integrate deserialized libraries better.
+abstract class LibraryProvider {
+  /// Looks up the library with the [canonicalUri].
+  LibraryElement lookupLibrary(Uri canonicalUri);
+}
+
 /// Handle for creating synthesized/patch libraries during library loading.
 abstract class LibraryLoader {
   /// This method must be called when a new synthesized/patch library has been