Handle private fields from mixins.

sdk/lib/_internal/compiler/implementation/elements/modelx.dart

Index: sdk/lib/_internal/compiler/implementation/elements/modelx.dart
diff --git a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
index 0a3b141791e79d26aac3b2b85613e227b0fb94de..8e9bf312151ad629658b80ebcdb76d50a272ddfa 100644
--- a/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
+++ b/sdk/lib/_internal/compiler/implementation/elements/modelx.dart
@@ -1530,7 +1530,7 @@ abstract class BaseClassElementX extends ElementX implements ClassElement {
   Element lookupSuperSelector(Selector selector) {
     return internalLookupSelector(selector, true);
   }
-  
+
   Element internalLookupSelector(Selector selector, bool isSuperLookup) {
     SourceString name = selector.name;
     bool isPrivate = name.isPrivate();
@@ -1583,24 +1583,25 @@ abstract class BaseClassElementX extends ElementX implements ClassElement {
    */
   bool isShadowedByField(Element fieldMember) {
     assert(fieldMember.isField());
-    // Note that we cannot use [lookupMember] or [lookupSuperMember] since it
-    // will not do the right thing for private elements.
-    ClassElement lookupClass = this;
+    SourceString fieldName = fieldMember.name;
+    bool isPrivate = fieldName.isPrivate();
     LibraryElement memberLibrary = fieldMember.getLibrary();
-    if (fieldMember.name.isPrivate()) {
-      // We find a super class in the same library as the field. This way the
-      // lookupMember will work.
-      while (lookupClass.getLibrary() != memberLibrary) {
-        lookupClass = lookupClass.superclass;
+    ClassElement lookupClass = this;
+    while (lookupClass != null) {
+      Element foundMember = lookupClass.lookupLocalMember(fieldName);
+      if (foundMember != null) {
+        if (foundMember == fieldMember) return false;
+        if (foundMember.isField()) {
+          if (!isPrivate || memberLibrary == foundMember.getLibrary()) {
+            // Private fields can only be shadowed by a field declared
+            // in the same library.
+            return true;
+          }
+        }
       }
+      lookupClass = lookupClass.superclass;
     }
-    SourceString fieldName = fieldMember.name;
-    while (true) {
-      Element foundMember = lookupClass.lookupMember(fieldName);
-      if (foundMember == fieldMember) return false;
-      if (foundMember.isField()) return true;
-      lookupClass = foundMember.getEnclosingClass().superclass;
-    }
+    return false;
   }
 
   Element validateConstructorLookupResults(Selector selector,