Initial support for mixins in dartc.

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

+// Copyright (c) 2013, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+package com.google.dart.compiler.resolver;
+
+/**
+ * Lexical scope corresponding to a mixin.
+ */
+class MixinScope extends Scope {
+  private final ClassElement classElement;
+
+  MixinScope(ClassElement classElement, Scope parent) {
+    super(classElement.getName(), parent.getLibrary(), parent);
+    this.classElement = classElement;
+  }
+
+  @Override
+  public Element declareElement(String name, Element element) {
+    throw new AssertionError("not supported yet");
+  }
+
+  @Override
+  public Element findLocalElement(String name) {
+    return Elements.findElement(classElement, name);
+  }
+}