[cfe] Don't expect external field to have no initializer

Change-Id: I146eff333466683b4375212397f6b54c8ff72aa9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/109262
Reviewed-by: Aske Simon Christensen <askesc@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

Author: johnniwinther

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

@@ -1588,10 +1588,6 @@ class ConstantEvaluator extends RecursiveVisitor<Constant> {
       final Member target = node.target;
       if (target is Field) {
         if (target.isConst) {
-          if (target.isInExternalLibrary && target.initializer == null) {
-            // The variable is unavailable due to separate compilation.
-            return unevaluated(node, new StaticGet(target));
-          }
           return runInsideContext(target, () {
             return _evaluateSubexpression(target.initializer);
           });