Issue 6711. Fix for NPE in resolver.

compiler/java/com/google/dart/compiler/resolver/Elements.java

Index: compiler/java/com/google/dart/compiler/resolver/Elements.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/Elements.java b/compiler/java/com/google/dart/compiler/resolver/Elements.java
index 06a2078c5b3bec7ff30ee5329f87ef4e607cad48..5fadc578d3218d1d457df7b0012f2b4d76d0b144 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -92,13 +92,26 @@ public class Elements {
   }
 
   public static LibraryElement getLibraryElement(Element element) {
-    do {
-      if (ElementKind.of(element).equals(ElementKind.LIBRARY)) {
-        break;
+    while (element != null) {
+      if (ElementKind.of(element) == ElementKind.LIBRARY) {
+        return (LibraryElement) element;
       }
-      element = element.getEnclosingElement();
-    } while (element != null && element.getEnclosingElement() != element);
-    return (LibraryElement) element;
+      EnclosingElement enclosingElement = element.getEnclosingElement();
+      if (enclosingElement == element) {
+        return null;
+      }
+      element = enclosingElement;
+    };
+    return null;
+  }
+  
+  /**
+   * @return <code>true</code> if "a" and "b" are declared in the same {@link LibraryElement}.
+   */
+  public static boolean areSameLibrary(Element a, Element b) {
+    LibraryElement aLibrary = getLibraryElement(a);
+    LibraryElement bLibrary = getLibraryElement(b);
+    return Objects.equal(aLibrary, bLibrary);
   }
 
   @VisibleForTesting