Complain about function literals not being constant expressions.

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

Index: pkg/front_end/lib/src/fasta/kernel/body_builder.dart
diff --git a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
index 42d3a59efa81f35d779ece3d562a0f71cce3c456..e868b64fd5bf7a2150c2afb1d0f3fca63ef8fac5 100644
--- a/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/body_builder.dart
@@ -2154,7 +2154,12 @@ class BodyBuilder extends ScopeListener<JumpTarget> implements BuilderHelper {
         typeParameters: typeParameters, asyncMarker: asyncModifier)
       ..fileOffset = beginToken.charOffset
       ..fileEndOffset = token.charOffset);
-    push(astFactory.functionExpression(function, beginToken));
+    if (constantExpressionRequired) {
+      push(buildCompileTimeError(
+          "Not a constant expression.", formals.charOffset));
+    } else {
+      push(astFactory.functionExpression(function, beginToken));
+    }
   }
 
   @override