Avoid patching in dart2dart.

pkg/compiler/lib/src/warnings.dart

Index: pkg/compiler/lib/src/warnings.dart
diff --git a/pkg/compiler/lib/src/warnings.dart b/pkg/compiler/lib/src/warnings.dart
index 8d78e10118e04894082d5acc46aaed580c413ee6..9b7dc994ce6f4866f75fc6db3e34d278da18bd43 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -2249,6 +2249,18 @@ main() sync* {
       "Cannot patch non-function with function patch "
       "'#{functionName}'.");
 
+  static const MessageKind EXTERNAL_WITH_BODY = const MessageKind(
+      "External function '#{functionName}' cannot have a function body.",
+      options: const ["--output-type=dart"],
+      howToFix: "Try removing the 'external' modifier or the function body.",
+      examples: const ["""
+external foo() => 0;
+main() => foo();
+""", """
+external foo() {}
+main() => foo();
+"""]);
+
   //////////////////////////////////////////////////////////////////////////////
   // Patch errors end.
   //////////////////////////////////////////////////////////////////////////////