Deal with deferred loading in the VM mirrors.

pkg/compiler/lib/src/mirrors/source_mirrors.dart

Index: pkg/compiler/lib/src/mirrors/source_mirrors.dart
diff --git a/pkg/compiler/lib/src/mirrors/source_mirrors.dart b/pkg/compiler/lib/src/mirrors/source_mirrors.dart
index 0afbd8e14b633177966a31cae49045eb3579861f..14e6c13994d0f7d84c37496cc448af6036957bd8 100644
--- a/pkg/compiler/lib/src/mirrors/source_mirrors.dart
+++ b/pkg/compiler/lib/src/mirrors/source_mirrors.dart
@@ -142,6 +142,10 @@ abstract class LibraryDependencySourceMirror
 
   /// Returns the source location for this import/export declaration.
   SourceLocation get location;
+
+  /// Returns a future that completes when the library is loaded and initates a
+  /// load if one has not already happened.
+  /*Future<LibraryMirror>*/ loadLibrary();
 }
 
 /// A mirror on a show/hide combinator declared on a library dependency.