Improve completion at the beginning of an index expression

This is almost identical to the fix for completing at the beginning of a
method invocation. There's probably a more consistent way to do this,
but I'm not sure what that is at the moment. (I think we used to do this
in `completionNode` but were skipping some nodes we shouldn't have been.)

Change-Id: I3f220e0e79a91721b0c1235fdfc0f511ee7400a9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/373902
Reviewed-by: Keerti Parthasarathy <keertip@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

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

@@ -1671,6 +1671,13 @@ class InScopeCompletionPass extends SimpleAstVisitor<void> {
 
   @override
   void visitIndexExpression(IndexExpression node) {
+    var beginToken = node.beginToken;
+    if (offset <= beginToken.end && beginToken.isKeywordOrIdentifier) {
+      // The user is completing at the beginning of the expression, so let the
+      // parent node determine the right set of suggestions.
+      node.parent?.accept(this);
+      return;
+    }
     if (offset >= node.leftBracket.end && offset <= node.rightBracket.offset) {
       collector.completionLocation = 'IndexExpression_index';
       _forExpression(node, mustBeNonVoid: true);

pkg/analysis_server/test/services/completion/dart/location/argument_list_test.dart

@@ -51,6 +51,33 @@ suggestions
 ''');
   }
 
+  Future<void> test_afterComma_beforeIndexExpression() async {
+    await computeSuggestions('''
+void f(List<int> l0) {
+  g(0, ^l0[1]);
+}
+void g(int i, int j) {}
+''');
+
+    assertResponse(r'''
+replacement
+  right: 2
+suggestions
+  l0
+    kind: parameter
+  true
+    kind: keyword
+  false
+    kind: keyword
+  null
+    kind: keyword
+  const
+    kind: keyword
+  switch
+    kind: keyword
+''');
+  }
+
   Future<void> test_afterComma_beforeMethodInvocation() async {
     allowedIdentifiers = {'random'};
     await computeSuggestions('''