Do not suggest the keyword const when completing in a constant context

While it isn't an error to use `const` in those contexts, it also isn't
necessary and we already omit `new` because it's never required. This
makes `const` more consistent with `new`.

Change-Id: Ia577065d53ab57543b86161f5e62e3947ff9a2a7
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/151029
Reviewed-by: Jaime Wren <jwren@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: bwilkerson

pkg/analysis_server/lib/src/services/completion/dart/keyword_contributor.dart

@@ -758,11 +758,13 @@ class _KeywordVisitor extends GeneralizingAstVisitor<void> {
 
   void _addExpressionKeywords(AstNode node) {
     _addSuggestions([
-      Keyword.CONST,
       Keyword.FALSE,
       Keyword.NULL,
       Keyword.TRUE,
     ]);
+    if (!request.inConstantContext) {
+      _addSuggestions([Keyword.CONST]);
+    }
     if (node.inClassMemberBody) {
       _addSuggestions([Keyword.SUPER, Keyword.THIS]);
     }