Complain about external methods with a body.

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

Index: pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart b/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
index 210e5a11886da11fda5cb602790742f26eeabbcf..0893ca26e3d1c8867e1a52ba21880bb69577b9db 100644
--- a/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/kernel_procedure_builder.dart
@@ -80,8 +80,14 @@ abstract class KernelFunctionBuilder
             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) {