Introduce a helper to emit import script tags.

lib/compiler/implementation/tree/unparser.dart

Index: lib/compiler/implementation/tree/unparser.dart
diff --git a/lib/compiler/implementation/tree/unparser.dart b/lib/compiler/implementation/tree/unparser.dart
index 6c199d83c51c92e24b926ef2515d20a8a7aebd3c..aabd65f369178d9c71f5494c63510c495f878723 100644
--- a/lib/compiler/implementation/tree/unparser.dart
+++ b/lib/compiler/implementation/tree/unparser.dart
@@ -24,11 +24,6 @@ class Unparser implements Visitor {
   }
   Unparser.withRenamer(this.rename) : sb = new StringBuffer();
 
-  // TODO(antonm): will go away soon.
-  void addString(String s) {
-    sb.add(s);
-  }
-
   void add(SourceString string) {
     string.printOn(sb);
   }
@@ -468,6 +463,11 @@ class Unparser implements Visitor {
     visit(node.statements);
   }
 
+  unparseImportTag(String uri, [String prefix]) {
+    final suffix = prefix === null ? '' : ',prefix:"$prefix"';
+    sb.add('#import("$uri"$suffix);');
+  }
+
   visitScriptTag(ScriptTag node) {
     add(node.beginToken.value);
     visit(node.tag);