Complain about external methods with a body.

pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart

 // Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library fasta.kernel_procedure_builder;
 
 import 'package:kernel/ast.dart'
     show
         Arguments,
         AsyncMarker,
@@ skipping 62 matching lines @@
       String name,
       List<TypeVariableBuilder> typeVariables,
       List<FormalParameterBuilder> formals,
       KernelLibraryBuilder compilationUnit,
       int charOffset,
       this.nativeMethodName)
       : super(metadata, modifiers, returnType, name, typeVariables, formals,
             compilationUnit, charOffset);
 
   void set body(Statement newBody) {
-    if (isAbstract && newBody != null) {
-      return internalError("Attempting to set body on abstract method.");
+    if (newBody != null) {
+      if (isAbstract) {
+        return internalError("Attempting to set body on abstract method.");
+      }
+      if (isExternal) {
+        return library.addCompileTimeError(
+            newBody.fileOffset, "An external method can't have a body.");
+      }
     }
     actualBody = newBody;
     if (function != null) {
       function.body = newBody;
       newBody?.parent = function;
     }
   }
 
   Statement get body => actualBody ??= new EmptyStatement();
 
@@ skipping 267 matching lines @@
         }
       }
       initializers.add(initializer..parent = constructor);
       initializers.add(superInitializer);
       return;
     }
     initializers.add(initializer);
     initializer.parent = constructor;
   }
 }