fix NSM when computing field formal parameter in non-constructor arg list

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

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library services.completion.contributor.dart.invocation;
 
 import 'dart:async';
 
 import 'package:analysis_server/src/services/completion/dart_completion_manager.dart';
 import 'package:analysis_server/src/services/completion/local_declaration_visitor.dart';
@@ skipping 73 matching lines @@
  * A suggestion builder for 'this.' constructor arguments.
  */
 class _FieldFormalSuggestionBuilder implements SuggestionBuilder {
   final DartCompletionRequest request;
 
   _FieldFormalSuggestionBuilder(this.request);
 
   @override
   bool computeFast(AstNode node) {
     if (node is FieldFormalParameter) {
-
-      // Compute fields already referenced
       ConstructorDeclaration constructorDecl =
           node.getAncestor((p) => p is ConstructorDeclaration);
-      List<String> referencedFields = new List<String>();
-      for (FormalParameter param in constructorDecl.parameters.parameters) {
-        if (param is FieldFormalParameter) {
-          SimpleIdentifier fieldId = param.identifier;
-          if (fieldId != null && fieldId != request.target.entity) {
-            String fieldName = fieldId.name;
-            if (fieldName != null && fieldName.length > 0) {
-              referencedFields.add(fieldName);
+      if (constructorDecl != null) {
+
+        // Compute fields already referenced
+        List<String> referencedFields = new List<String>();
+        for (FormalParameter param in constructorDecl.parameters.parameters) {
+          if (param is FieldFormalParameter) {
+            SimpleIdentifier fieldId = param.identifier;
+            if (fieldId != null && fieldId != request.target.entity) {
+              String fieldName = fieldId.name;
+              if (fieldName != null && fieldName.length > 0) {
+                referencedFields.add(fieldName);
+              }
             }
           }
         }
-      }
 
-      // Add suggestions for fields that are not already referenced
-      ClassDeclaration classDecl =
-          constructorDecl.getAncestor((p) => p is ClassDeclaration);
-      for (ClassMember member in classDecl.members) {
-        if (member is FieldDeclaration) {
-          for (VariableDeclaration varDecl in member.fields.variables) {
-            SimpleIdentifier fieldId = varDecl.name;
-            if (fieldId != null) {
-              String fieldName = fieldId.name;
-              if (fieldName != null && fieldName.length > 0) {
-                if (!referencedFields.contains(fieldName)) {
-                  CompletionSuggestion suggestion =
-                      createFieldSuggestion(member, varDecl);
-                  if (suggestion != null) {
-                    request.addSuggestion(suggestion);
+        // Add suggestions for fields that are not already referenced
+        ClassDeclaration classDecl =
+            constructorDecl.getAncestor((p) => p is ClassDeclaration);
+        for (ClassMember member in classDecl.members) {
+          if (member is FieldDeclaration) {
+            for (VariableDeclaration varDecl in member.fields.variables) {
+              SimpleIdentifier fieldId = varDecl.name;
+              if (fieldId != null) {
+                String fieldName = fieldId.name;
+                if (fieldName != null && fieldName.length > 0) {
+                  if (!referencedFields.contains(fieldName)) {
+                    CompletionSuggestion suggestion =
+                        createFieldSuggestion(member, varDecl);
+                    if (suggestion != null) {
+                      request.addSuggestion(suggestion);
+                    }
                   }
                 }
               }
             }
           }
         }
       }
     } else {
       // This should never be called with a case not handled above.
       assert(false);
@@ skipping 287 matching lines @@
     }
     return new Future.value(false);
   }
 
   @override
   Future<bool> visitVariableElement(VariableElement element) {
     InterfaceTypeSuggestionBuilder.suggestionsFor(request, element.type);
     return new Future.value(true);
   }
 }