Add LibraryDependencyMirror+CombinatorMirror to source mirrors.

sdk/lib/_internal/compiler/implementation/tree/nodes.dart

Index: sdk/lib/_internal/compiler/implementation/tree/nodes.dart
diff --git a/sdk/lib/_internal/compiler/implementation/tree/nodes.dart b/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
index 604ef462f7c4b4741371755d9c754b08464c936e..84ae064d7a671452ff0edef70a7f324479324ba2 100644
--- a/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree/nodes.dart
@@ -162,6 +162,7 @@ abstract class Node extends TreeElementMixin implements Spannable {
   Label asLabel() => null;
   LabeledStatement asLabeledStatement() => null;
   LibraryName asLibraryName() => null;
+  LibraryDependency asLibraryDependency() => null;
   LiteralBool asLiteralBool() => null;
   LiteralDouble asLiteralDouble() => null;
   LiteralInt asLiteralInt() => null;
@@ -1777,6 +1778,8 @@ abstract class LibraryDependency extends LibraryTag {
                     this.combinators,
                     Link<MetadataAnnotation> metadata)
     : super(metadata);
+
+  LibraryDependency asLibraryDependency() => this;
 }
 
 /**