Refactor Library/CompilationUnit and how we define local Scope.

lib/compiler/implementation/resolver.dart

 // Copyright (c) 2012, 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.
 
 interface TreeElements {
   Element operator[](Node node);
   Selector getSelector(Send send);
   Type getType(TypeAnnotation annotation);
 }
 
@@ skipping 1866 matching lines @@
     } else {
       cls.allSupertypes = const EmptyLink<Type>();
     }
   }
 
   /**
    * Add a synthetic nullary constructor if there are no other
    * constructors.
    */
   void addDefaultConstructorIfNeeded(ClassElement element) {
-    if (element.constructors.length != 0) return;
+    if (element.hasConstructor) return;
     SynthesizedConstructorElement constructor =
       new SynthesizedConstructorElement(element);
-    element.constructors[element.name] = constructor;
+    element.addToScope(constructor, compiler);
     Type returnType = compiler.types.voidType;
     constructor.type = new FunctionType(returnType, const EmptyLink<Type>(),
                                         constructor);
     constructor.cachedNode =
       new FunctionExpression(new Identifier(element.position()),
                              new NodeList.empty(),
                              new Block(new NodeList.empty()),
                              null, null, null, null);
   }
 
@@ skipping 409 matching lines @@
   TopScope(LibraryElement library) : super(null, library);
   Element lookup(SourceString name) {
     return library.find(name);
   }
 
   Element add(Element newElement) {
     throw "Cannot add an element in the top scope";
   }
   String toString() => '$element';
 }