Issue 3266. Produce a warning when trying to call an inaccessible method

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 7325b798ef5a4758ac8ad5b36c8c21a4c180d192..da7c156d409a7a0eefc44b7248c0d3cfd242e13b 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -36,6 +36,7 @@ import com.google.dart.compiler.type.InterfaceType;
 import com.google.dart.compiler.type.Type;
 import com.google.dart.compiler.type.TypeVariable;
 import com.google.dart.compiler.util.Paths;
+import com.google.dart.compiler.util.apache.StringUtils;
 
 import java.io.File;
 import java.net.URI;
@@ -605,6 +606,29 @@ static FieldElementImplementation fieldFromNode(DartField node,
     }
     return false;
   }
+
+  /**
+   * @return the {@link LibraryElement} which declares given {@link Element}.
+   */
+  public static LibraryElement getDeclaringLibrary(Element element) {
+    while (element != null) {
+      if (element instanceof LibraryElement) {
+        return (LibraryElement) element;
+      }
+      element = element.getEnclosingElement();
+    }
+    return null;
+  }
+  
+  /**
+   * @return <code>true</code> if "element" is accessible in "scopeLibrary".
+   */
+  public static boolean isAccessible(LibraryElement scopeLibrary, Element element) {
+    if (element != null && StringUtils.startsWith(element.getName(), "_")) {
+      return Objects.equal(scopeLibrary, getDeclaringLibrary(element));
+    }
+    return true;
+  }
   
   /**
    * Looks to see if the property access requires a getter.