fix completion after dot-keyword - fixes #23882

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

 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/utilities_dart.dart';
 
 int _computeArgIndex(AstNode containingNode, Object entity) {
   var argList = containingNode;
   if (argList is ArgumentList) {
     NodeList<Expression> args = argList.arguments;
     for (int index = 0; index < args.length; ++index) {
@@ skipping 302 matching lines @@
     // A token is considered a candidate entity if the cursor offset is (a)
     // before the start of the token, (b) within the token, (c) at the end of
     // the token and the token is a keyword or identifier, or (d) at the
     // location of the token and the token is zero length.
     if (offset < token.end) {
       return true;
     } else if (offset == token.end) {
       return token.type == TokenType.KEYWORD ||
           token.type == TokenType.IDENTIFIER ||
           token.length == 0;
+    } else if (!token.isSynthetic) {
+      return false;
+    }
+    // If the current token is synthetic, then check the previous token
+    // because it may have been dropped from the parse tree
+    Token previous = token.previous;
+    if (offset < previous.end) {
+      return true;
+    } else if (offset == previous.end) {
+      return token.type == TokenType.KEYWORD ||
+          previous.type == TokenType.IDENTIFIER;
     } else {
       return false;
     }
   }
 
   /**
    * Return `true` if the parameter is a functional parameter.
    */
   static bool _isFunctionalParameter(
       List<ParameterElement> parameters, int paramIndex) {
     if (paramIndex < parameters.length) {
       ParameterElement param = parameters[paramIndex];
       DartType paramType = param.type;
       if (param.parameterKind == ParameterKind.NAMED) {
         // TODO(danrubel) handle named parameters
         return false;
       } else {
         return paramType is FunctionType || paramType is FunctionTypeAlias;
       }
     }
     return false;
   }
 }