Avoid patching in dart2dart.

pkg/compiler/lib/src/elements/modelx.dart

Index: pkg/compiler/lib/src/elements/modelx.dart
diff --git a/pkg/compiler/lib/src/elements/modelx.dart b/pkg/compiler/lib/src/elements/modelx.dart
index fc70d65018412a1c350d8943128be50a6648e7c4..45e1f955bfbf39285d3f0604352149564112d2ed 100644
--- a/pkg/compiler/lib/src/elements/modelx.dart
+++ b/pkg/compiler/lib/src/elements/modelx.dart
@@ -118,8 +118,6 @@ abstract class ElementX extends Element {
 
   bool get isSynthesized => false;
 
-  bool get isForwardingConstructor => false;
-
   bool get isMixinApplication => false;
 
   bool get isLocal => false;
@@ -306,6 +304,7 @@ class ErroneousElementX extends ElementX implements ErroneousElement {
   get nestedClosures => unsupported();
   get memberContext => unsupported();
   get executableContext => unsupported();
+  get isExternal => unsupported();
 
   bool get isRedirectingFactory => unsupported();
 
@@ -1557,6 +1556,8 @@ abstract class BaseFunctionElementX
     assert(modifiers != null);
   }
 
+  bool get isExternal => modifiers.isExternal;
+
   bool get isInstanceMember {
     return isClassMember
            && !isConstructor