[cfe] Allow expression compilation with constant-update-2018

Allow expressions to be compiled by the VM but without evaluating
constants.  This will use the VM's C++ constant evaluator.

Fixes #36479

Change-Id: Icd6e769e53db5d211a083d1f5cff7ddf88cc4da2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/98675
Commit-Queue: Kevin Millikin <kmillikin@google.com>
Reviewed-by: Aske Simon Christensen <askesc@google.com>

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

@@ -70,7 +70,7 @@ import '../messages.dart'
         templateMissingImplementationCause,
         templateSuperclassHasNoDefaultConstructor;
 
-import '../problems.dart' show unhandled, unimplemented;
+import '../problems.dart' show unhandled;
 
 import '../scope.dart' show AmbiguousBuilder;
 
@@ -790,10 +790,6 @@ class KernelTarget extends TargetImplementation {
   }
 
   void runProcedureTransformations(Procedure procedure) {
-    if (loader.target.enableConstantUpdate2018) {
-      unimplemented('constant evaluation during expression evaluation',
-          procedure.fileOffset, procedure.fileUri);
-    }
     backendTarget.performTransformationsOnProcedure(
         loader.coreTypes, loader.hierarchy, procedure,
         logger: (String msg) => ticker.logMs(msg));