Added support for exports and re-exports.

lib/compiler/implementation/tree/nodes.dart

Index: lib/compiler/implementation/tree/nodes.dart
diff --git a/lib/compiler/implementation/tree/nodes.dart b/lib/compiler/implementation/tree/nodes.dart
index 5fcd0eadcd7cd09d3b79318adc5105c1a4113e26..3e90f60cd0ffedf4ae90486482eef4168a99b479 100644
--- a/lib/compiler/implementation/tree/nodes.dart
+++ b/lib/compiler/implementation/tree/nodes.dart
@@ -1680,14 +1680,32 @@ class LibraryName extends LibraryTag {
   Token getEndToken() => name.getEndToken().next;
 }
 
-class Import extends LibraryTag {
+/**
+ * This tag describes a dependency between one library and the exported
+ * identifiers of another library. The other library is specified by the [uri].
+ * Combinators filter away some identifiers from the other library.
+ */
+abstract class LibraryDependency extends LibraryTag {
   final LiteralString uri;
-  final Identifier prefix;
   final NodeList combinators;
 
+  LibraryDependency(this.uri, this.combinators);
+}
+
+/**
+ * An [:import:] library tag.
+ *
+ * An import tag is dependency on another library where the exported identifiers
+ * are put into the import scope of the importing library. The import scope is
+ * only visible inside the library.
+ */
+class Import extends LibraryDependency {
+  final Identifier prefix;
   final Token importKeyword;
 
-  Import(this.importKeyword, this.uri, this.prefix, this.combinators);
+  Import(this.importKeyword, LiteralString uri,
+         this.prefix, NodeList combinators)
+      : super(uri, combinators);
 
   bool get isImport => true;
 
@@ -1712,13 +1730,18 @@ class Import extends LibraryTag {
   }
 }
 
-class Export extends LibraryTag {
-  final LiteralString uri;
-  final NodeList combinators;
-
+/**
+ * An [:export:] library tag.
+ *
+ * An export tag is dependency on another library where the exported identifiers
+ * are put into the export scope of the exporting library. The export scope is
+ * not visible inside the library.
+ */
+class Export extends LibraryDependency {
   final Token exportKeyword;
 
-  Export(this.exportKeyword, this.uri, this.combinators);
+  Export(this.exportKeyword, LiteralString uri, NodeList combinators)
+      : super(uri, combinators);
 
   bool get isExport => true;