Make method overrides with differing parameters a compile-time error

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 906407103377a90da773e4eb67aeb09815ce2695..f9a37350ce250fca438fc8153af8486ae255b760 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -56,6 +56,16 @@ public class Elements {
     return new LibraryElementImplementation(libraryUnit);
   }
 
+  public static LibraryElement getLibraryElement(Element element) {
+    do {
+      if (ElementKind.of(element).equals(ElementKind.LIBRARY)) {
+        break;
+      }
+      element = element.getEnclosingElement();
+    } while (element != null && element.getEnclosingElement() != element);
+    return (LibraryElement) element;
+  }
+
   @VisibleForTesting
   public static MethodElement methodElement(DartFunctionExpression node, String name) {
     return new MethodElementImplementation(node, name, Modifiers.NONE);