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

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

Index: compiler/java/com/google/dart/compiler/resolver/Elements.java
diff --git a/compiler/java/com/google/dart/compiler/resolver/Elements.java b/compiler/java/com/google/dart/compiler/resolver/Elements.java
index 41ac3cdfb077cd07cb7b4d497257b69bc24a0f74..846b420a6a3c3e337c7849102b05c43e535aa70d 100644
--- a/compiler/java/com/google/dart/compiler/resolver/Elements.java
+++ b/compiler/java/com/google/dart/compiler/resolver/Elements.java
@@ -169,7 +169,10 @@ public class Elements {
   }
 
   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) {