Make some ErrorCode-s compile-time errors and some just type warnings

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

 // Copyright (c) 2011, 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;
 
 import com.google.common.annotations.VisibleForTesting;
 import com.google.dart.compiler.ast.DartClass;
 import com.google.dart.compiler.ast.DartField;
 import com.google.dart.compiler.ast.DartFunctionExpression;
@@ skipping 151 matching lines @@
 
   public static DynamicElement dynamicElement() {
     return DynamicElementImplementation.getInstance();
   }
 
   static ConstructorElement lookupConstructor(ClassElement cls, ClassElement type, String name) {
     return ((ClassElementImplementation) cls).lookupConstructor(type, name);
   }
 
   static ConstructorElement lookupConstructor(ClassElement cls, String name) {
-    return ((ClassElementImplementation) cls).lookupConstructor(name);
+    if (cls instanceof  ClassElementImplementation) {
+      return ((ClassElementImplementation) cls).lookupConstructor(name);
+    }
+    return null;
   }
 
   public static MethodElement lookupLocalMethod(ClassElement cls, String name) {
     return ((ClassElementImplementation) cls).lookupLocalMethod(name);
   }
 
   public static FieldElement lookupLocalField(ClassElement cls, String name) {
     return ((ClassElementImplementation) cls).lookupLocalField(name);
   }
 
@@ skipping 34 matching lines @@
     return VoidElement.getInstance();
   }
 
   /**
    * Returns true if the class needs an implicit default constructor.
    */
   public static boolean needsImplicitDefaultConstructor(ClassElement classElement) {
     return !classElement.isObject() && classElement.getConstructors().isEmpty();
   }
 }