Handle creation of class members in files other than the file containing the error (issue 30327)

pkg/analysis_server/lib/src/services/correction/fix_internal.dart

Index: pkg/analysis_server/lib/src/services/correction/fix_internal.dart
diff --git a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
index 3accd66aef277792cc2b45ff0a7fb41c4c3def71..e25e759af336bc0d06d96fb26618bb6ba8fe76ff 100644
--- a/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
+++ b/pkg/analysis_server/lib/src/services/correction/fix_internal.dart
@@ -1167,7 +1167,7 @@ class FixProcessor {
     ClassDeclaration targetClassNode = targetTypeNode;
     // prepare location
     ClassMemberLocation targetLocation =
-        utils.prepareNewFieldLocation(targetClassNode);
+        _getUtilsFor(targetClassNode).prepareNewFieldLocation(targetClassNode);
     // build field source
     Source targetSource = targetClassElement.source;
     String targetFile = targetSource.fullName;
@@ -1324,7 +1324,7 @@ class FixProcessor {
     ClassDeclaration targetClassNode = targetTypeNode;
     // prepare location
     ClassMemberLocation targetLocation =
-        utils.prepareNewGetterLocation(targetClassNode);
+        _getUtilsFor(targetClassNode).prepareNewGetterLocation(targetClassNode);
     // build method source
     Source targetSource = targetClassElement.source;
     String targetFile = targetSource.fullName;
@@ -2866,6 +2866,26 @@ class FixProcessor {
   }
 
   /**
+   * Return the correction utilities that should be used when creating an edit
+   * in the compilation unit containing the given [node].
+   */
+  CorrectionUtils _getUtilsFor(AstNode node) {
+    CompilationUnit targetUnit =
+        node.getAncestor((node) => node is CompilationUnit);
+    CompilationUnitElement targetUnitElement = targetUnit?.element;
+    CorrectionUtils realUtils = utils;
+    if (targetUnitElement != utils.unit.element) {
+      realUtils = new CorrectionUtils(targetUnit);
+      ClassDeclaration targetClass =
+          node.getAncestor((node) => node is ClassDeclaration);
+      if (targetClass != null) {
+        realUtils.targetClassElement = targetClass.element;
+      }
+    }
+    return realUtils;
+  }
+
+  /**
    * Returns an expected [DartType] of [expression], may be `null` if cannot be
    * inferred.
    */