Fix insertion of imports when there are no existing directives (issue 30430)

pkg/analysis_server/lib/src/computer/import_elements_computer.dart

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:analysis_server/protocol/protocol_generated.dart';
 import 'package:analyzer/dart/analysis/results.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/ast_factory.dart';
@@ skipping 59 matching lines @@
         if (matchingImports.isEmpty) {
           //
           // The required library is not being imported with a matching prefix,
           // so we need to add an import.
           //
           File importedFile = resourceProvider.getFile(importedElements.path);
           Uri uri = sourceFactory.restoreUri(importedFile.createSource());
           Source importedSource = importedFile.createSource(uri);
           String importUri =
               _getLibrarySourceUri(libraryElement, importedSource);
-          int offset = _offsetForInsertion(importUri);
-          builder.addInsertion(offset, (DartEditBuilder builder) {
-            builder.writeln();
+          _InsertionDescription description =
+              _getInsertionDescription(importUri);
+          builder.addInsertion(description.offset, (DartEditBuilder builder) {
+            for (int i = 0; i < description.newLinesBefore; i++) {
+              builder.writeln();
+            }
             builder.write("import '");
             builder.write(importUri);
             builder.write("'");
             if (importedElements.prefix.isNotEmpty) {
               builder.write(' as ');
               builder.write(importedElements.prefix);
             }
             builder.write(';');
+            for (int i = 0; i < description.newLinesAfter; i++) {
+              builder.writeln();
+            }
           });
         } else {
           //
           // There are some imports of the library with a matching prefix. We
           // need to determine whether the names are already visible or whether
           // we need to make edits to make them visible.
           //
           // Compute the edits that need to be made.
           //
           Map<ImportDirective, _ImportUpdate> updateMap =
@@ skipping 201 matching lines @@
     List<ImportDirective> matchingImports = <ImportDirective>[];
     for (ImportDirective existingImport in existingImports) {
       if (_matches(existingImport, importedElements)) {
         matchingImports.add(existingImport);
       }
     }
     return matchingImports;
   }
 
   /**
+   * Return the offset at which an import of the given [importUri] should be
+   * inserted.
+   *
+   * Partially copied from DartFileEditBuilderImpl.
+   */
+  _InsertionDescription _getInsertionDescription(String importUri) {
+    CompilationUnit unit = libraryResult.unit;
+    LibraryDirective libraryDirective;
+    List<ImportDirective> importDirectives = <ImportDirective>[];
+    List<Directive> otherDirectives = <Directive>[];
+    for (Directive directive in unit.directives) {
+      if (directive is LibraryDirective) {
+        libraryDirective = directive;
+      } else if (directive is ImportDirective) {
+        importDirectives.add(directive);
+      } else {
+        otherDirectives.add(directive);
+      }
+    }
+    if (importDirectives.isEmpty) {
+      if (libraryDirective == null) {
+        if (otherDirectives.isEmpty) {
+          // TODO(brianwilkerson) Insert after any non-doc comments.
+          return new _InsertionDescription(0, after: 2);
+        }
+        return new _InsertionDescription(otherDirectives[0].offset, after: 2);
+      }
+      return new _InsertionDescription(libraryDirective.end, before: 2);
+    }
+    // TODO(brianwilkerson) Fix this to find the right location.
+    // See DartFileEditBuilderImpl._addLibraryImports for inspiration.
+    return new _InsertionDescription(importDirectives.last.end, before: 1);
+  }
+
+  /**
    * Computes the best URI to import [what] into [from].
    *
    * Copied from DartFileEditBuilderImpl.
    */
   String _getLibrarySourceUri(LibraryElement from, Source what) {
     String whatPath = what.fullName;
     // check if an absolute URI (such as 'dart:' or 'package:')
     Uri whatUri = what.uri;
     String whatUriScheme = whatUri.scheme;
     if (whatUriScheme != '' && whatUriScheme != 'file') {
       return whatUri.toString();
     }
     // compute a relative URI
     Context context = resourceProvider.pathContext;
     String fromFolder = context.dirname(from.source.fullName);
     String relativeFile = context.relative(whatPath, from: fromFolder);
     return context.split(relativeFile).join('/');
   }
 
   /**
    * Return `true` if the given [import] matches the given specification of
    * [importedElements]. They will match if they import the same library using
    * the same prefix.
    */
   bool _matches(ImportDirective import, ImportedElements importedElements) {
     return (import.element as ImportElement).importedLibrary.source.fullName ==
             importedElements.path &&
         (import.prefix?.name ?? '') == importedElements.prefix;
   }
-
-  /**
-   * Return the offset at which an import of the given [importUri] should be
-   * inserted.
-   *
-   * Partially copied from DartFileEditBuilderImpl.
-   */
-  int _offsetForInsertion(String importUri) {
-    // TODO(brianwilkerson) Fix this to find the right location.
-    // See DartFileEditBuilderImpl._addLibraryImports for inspiration.
-    CompilationUnit unit = libraryResult.unit;
-    LibraryDirective libraryDirective;
-    List<ImportDirective> importDirectives = <ImportDirective>[];
-    for (Directive directive in unit.directives) {
-      if (directive is LibraryDirective) {
-        libraryDirective = directive;
-      } else if (directive is ImportDirective) {
-        importDirectives.add(directive);
-      }
-    }
-    if (importDirectives.isEmpty) {
-      if (libraryDirective == null) {
-        return 0;
-      }
-      return libraryDirective.end;
-    }
-    return importDirectives.last.end;
-  }
 }
 
 /**
  * Information about how a given import directive needs to be updated in order
  * to make the required names visible.
  */
 class _ImportUpdate {
   /**
    * The import directive to be updated.
    */
@@ skipping 22 matching lines @@
     namesToShow.add(name);
   }
 
   /**
    * Record that the given [name] needs to be removed from hide combinators.
    */
   void unhide(String name) {
     namesToUnhide.add(name);
   }
 }
+
+class _InsertionDescription {
+  final int newLinesBefore;
+  final int offset;
+  final int newLinesAfter;
+
+  _InsertionDescription(this.offset, {int before: 0, int after: 0})
+      : this.newLinesBefore = before,
+        this.newLinesAfter = after;
+}