Add compile-time errors for enums.

pkg/compiler/lib/src/dart_types.dart

Index: pkg/compiler/lib/src/dart_types.dart
diff --git a/pkg/compiler/lib/src/dart_types.dart b/pkg/compiler/lib/src/dart_types.dart
index 32acb8871d16783a3ea8d8d2c03dddcc8d2064c7..22d949d3a290ac9c73cbc6b23d43897a368bed86 100644
--- a/pkg/compiler/lib/src/dart_types.dart
+++ b/pkg/compiler/lib/src/dart_types.dart
@@ -127,6 +127,9 @@ abstract class DartType {
   /// Is [: true :] if this type is a malformed type.
   bool get isMalformed => kind == TypeKind.MALFORMED_TYPE;
 
+  /// Is `true` if this type is declared by an enum.
+  bool get isEnumType => false;
+
   /// Returns an occurrence of a type variable within this type, if any.
   TypeVariableType get typeVariableOccurrence => null;
 
@@ -461,6 +464,8 @@ class InterfaceType extends GenericType {
 
   bool get isObject => element.isObject;
 
+  bool get isEnumType => element.isEnumClass;
+
   InterfaceType createInstantiation(List<DartType> newTypeArguments) {
     return new InterfaceType(element, newTypeArguments);
   }