Complain about external methods with a body.

pkg/front_end/lib/src/fasta/kernel/body_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.body_builder;
 
 import '../fasta_codes.dart'
     show FastaMessage, codeExpectedButGot, codeExpectedFunctionBody;
 
 import '../parser/parser.dart'
@@ skipping 466 matching lines @@
       initializer =
           buildSuperInitializer(node.target, node.arguments, token.charOffset);
     } else {
       Expression value = toValue(node);
       if (node is! Throw) {
         value = wrapInCompileTimeError(value, "Expected an initializer.");
       }
       initializer = buildInvalidInitializer(node, token.charOffset);
     }
     _typeInferrer.inferInitializer(initializer);
-    if (member is KernelConstructorBuilder) {
+    if (member is KernelConstructorBuilder && !member.isExternal) {
       member.addInitializer(initializer);
     } else {
       addCompileTimeError(
           token.charOffset, "Can't have initializers: ${member.name}");
     }
   }
 
   DartType _computeReturnTypeContext(MemberBuilder member) {
     if (member is KernelProcedureBuilder) {
       return member.target.function.returnType;
@@ skipping 2975 matching lines @@
     if (starToken == null) {
       return AsyncMarker.Async;
     } else {
       assert(identical(starToken.stringValue, "*"));
       return AsyncMarker.AsyncStar;
     }
   } else {
     return internalError("Unknown async modifier: $asyncToken");
   }
 }