Issue 2693. Don't add null imports

compiler/java/com/google/dart/compiler/ast/LibraryUnit.java

Index: compiler/java/com/google/dart/compiler/ast/LibraryUnit.java
diff --git a/compiler/java/com/google/dart/compiler/ast/LibraryUnit.java b/compiler/java/com/google/dart/compiler/ast/LibraryUnit.java
index a9c0eab9bff18cab14fd8a06069eade87aed9357..d5e7feb4594a13c507d0dd5c6da6bb01414d999b 100644
--- a/compiler/java/com/google/dart/compiler/ast/LibraryUnit.java
+++ b/compiler/java/com/google/dart/compiler/ast/LibraryUnit.java
@@ -1,4 +1,4 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
@@ -111,9 +111,11 @@ public class LibraryUnit {
   }
 
   public void addImport(LibraryUnit unit, LibraryNode node) {
-    imports.add(unit);
-    if (node != null && node.getPrefix() != null) {
-      prefixes.put(unit, node.getPrefix());
+    if (unit != null) {
+      imports.add(unit);
+      if (node != null && node.getPrefix() != null) {
+        prefixes.put(unit, node.getPrefix());
+      }
     }
   }