Reduce strong mode errors and warnings

pkg/front_end/lib/src/fasta/source/outline_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.outline_builder;
 
 import 'package:kernel/ast.dart' show
     AsyncMarker,
     ProcedureKind;
 
@@ skipping 334 matching lines @@
     List parameters = popList(count);
     for (FormalParameterBuilder parameter in parameters) {
       parameter.kind = kind;
     }
     push(parameters);
   }
 
   @override
   void endFormalParameters(int count, Token beginToken, Token endToken) {
     debugEvent("FormalParameters");
+    // TODO(sigmund, ahe): to make strong clean we can either cast every value
+    // below, or split this list in 2, and simply ensure that we always have a
+    // list for optional arguments (empty or null there are none):
+    //   List<FormalParameterBuilder> optionalFormals = pop();
+    //   List<FormalParameterBuilder> requiredFormals = popList(count);
     List formals = popList(count);
     if (formals != null && formals.isNotEmpty) {
       var last = formals.last;
       if (last is List) {
+        List<FormalParameterBuilder> l = last;

[Siggi Cherem (dart-lang), 2017/02/02 04:26:58]

I don't want to submit this, this was just a test to see if I could get rid of
the errors. I think it's more viable to do something like what I propose above
so we get rid of this implicit union type here.

+        List<FormalParameterBuilder> f = formals.sublist(0, formals.length - 1);
         var newList =
             new List<FormalParameterBuilder>(formals.length - 1 + last.length);
-        newList.setRange(0, formals.length - 1, formals);
-        newList.setRange(formals.length - 1, newList.length, last);
+        newList.setRange(0, formals.length - 1, f);
+        newList.setRange(formals.length - 1, newList.length, l);
         for (int i = 0; i < last.length; i++) {
           newList[i + formals.length - 1] = last[i];
         }
         formals = newList;
       }
     }
     if (formals != null) {
       for (var formal in formals) {
         if (formal is! FormalParameterBuilder) {
           internalError(formals);
@@ skipping 149 matching lines @@
   void handleModifiers(int count) {
     debugEvent("Modifiers");
     push(popList(count) ?? NullValue.Modifiers);
   }
 
   @override
   void debugEvent(String name) {
     // printEvent(name);
   }
 }