Support imports for dart: libs.

lib/compiler/implementation/dart_backend/emitter.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Dart backend helper for converting program IR back to source code.
  */
 class Emitter {
 
   final Compiler compiler;
   final StringBuffer sb;
-  final Renamer renamer;
+  final ConflictingRenamer renamer;
 
   Emitter(Compiler compiler) :
       this.compiler = compiler,
       sb = new StringBuffer(),
       renamer = new ConflictingRenamer(compiler);
 
   /**
    * Outputs given class element with selected inner elements.
    */
   void outputClass(ClassElement classElement, Set<Element> innerElements) {
@@ skipping 42 matching lines @@
     if (element is SynthesizedConstructorElement
         || element is AbstractFieldElement) return;
     if (element.isField()) {
       assert(element is VariableElement);
       sb.add(unparser.unparse(element.variables.parseNode(compiler)));
     } else {
       sb.add(unparser.unparse(element.parseNode(compiler)));
     }
   }
 
-  String toString() => sb.toString();
+  String toString() {
+    final result = new StringBuffer();
+    final libraries = compiler.libraries;
+    for (final uri in libraries.getKeys()) {
+      // Same library element may be a value for different uris as of now
+      // e.g., core libraryElement is a value for both keys 'dart:core'
+      // and full file name.  Only care about uris with dart scheme.
+      if (!uri.startsWith('dart:')) continue;
+      final lib = libraries[uri];
+      if (renamer.imports.containsKey(lib)) {
+        result.add('#import("$uri", prefix: "${renamer.imports[lib]}");');
+      }
+    }
+    result.add(sb);
+    return result.toString();
+  }
 }