Report external without patch as an error.

sdk/lib/_internal/compiler/implementation/scanner/listener.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/listener.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
index 8f1b67529b608fda54b9c3e51ffc616e7e6b71ca..216c9286da3aca43de771275fbb19c86a704516b 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
@@ -1909,8 +1909,10 @@ class PartialFunctionElement extends FunctionElementX {
     if (cachedNode != null) return cachedNode;
     if (patch == null) {
       if (modifiers.isExternal()) {
-        listener.cancel("External method without an implementation",
-                        element: this);
+        listener.reportMessage(
+            listener.spanFromSpannable(this),
+            MessageKind.EXTERNAL_WITHOUT_IMPLEMENTATION.error(),
+            api_s.Diagnostic.ERROR);
       }
     }
     parseFunction(Parser p) {