Fix field-accesses for private fields that were "shadowed" by other private fields.

lib/compiler/implementation/world.dart

Index: lib/compiler/implementation/world.dart
diff --git a/lib/compiler/implementation/world.dart b/lib/compiler/implementation/world.dart
index 03e4e3a1906aa52d95b2ffbe54361c1c4aef6f59..38615cca942e177f64673b850199e8a4c06ed74f 100644
--- a/lib/compiler/implementation/world.dart
+++ b/lib/compiler/implementation/world.dart
@@ -29,20 +29,25 @@ class World {
   }
 
   /**
-   * Returns a [MemberSet] that contains the possible targets of a
-   * selector named [member] on a receiver whose type is [type].
+   * Returns a [MemberSet] that contains the possible targets of the given
+   * [selector] on a receiver with the given [type].
    */
-  MemberSet _memberSetFor(Type type, SourceString member) {
+  MemberSet _memberSetFor(Type type, Selector selector) {
     assert(compiler !== null);
     ClassElement cls = type.element;
-    MemberSet result = new MemberSet(member);
-    Element element = cls.lookupMember(member);
+    SourceString name = selector.name;
+    LibraryElement library = selector.library;
+    MemberSet result = new MemberSet(name);
+    Element element = cls.lookupSelector(selector);
     if (element !== null) result.add(element);
 
+    bool isPrivate = name.isPrivate();
     Set<ClassElement> subtypesOfCls = subtypes[cls];
     if (subtypesOfCls !== null) {
       for (ClassElement sub in subtypesOfCls) {
-        element = sub.lookupLocalMember(member);
+        // Private members from a different library are not visible.
+        if (isPrivate && sub.getLibrary() != library) continue;
+        element = sub.lookupLocalMember(name);
         if (element !== null) result.add(element);
       }
     }
@@ -50,11 +55,11 @@ class World {
   }
 
   /**
-   * Returns the single field with the given name, if such a field
-   * exists. If there are multple fields, or none, return null.
+   * Returns the single field with the given [selector]. If there is no such
+   * field, or there are multiple possible fields returns [:null:].
    */
-  VariableElement locateSingleField(Type type, SourceString member) {
-    MemberSet memberSet = _memberSetFor(type, member);
+  VariableElement locateSingleField(Type type, Selector selector) {
+    MemberSet memberSet = _memberSetFor(type, selector);
     int fieldCount = 0;
     int nonFieldCount = 0;
     VariableElement field;
@@ -71,8 +76,8 @@ class World {
 
   Set<ClassElement> findNoSuchMethodHolders(Type type) {
     Set<ClassElement> result = new Set<ClassElement>();
-    MemberSet memberSet = _memberSetFor(type, Compiler.NO_SUCH_METHOD);
     Selector noSuchMethodSelector = new Selector.noSuchMethod();
+    MemberSet memberSet = _memberSetFor(type, noSuchMethodSelector);
     for (Element element in memberSet.elements) {
       ClassElement holder = element.getEnclosingClass();
       if (holder !== compiler.objectClass &&