statement completion for expression bodies

See: https://github.com/flutter/flutter-intellij/issues/3253

Change-Id: Ib892833ecfa4500b726f4f6df42f16f0618c994d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/118663
Reviewed-by: Steve Messick <messick@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

pkg/analysis_server/lib/src/services/completion/statement/statement_completion.dart

@@ -207,8 +207,9 @@ class StatementCompletionProcessor {
     } else if (node is Declaration) {
       if (errors.isNotEmpty) {
         if (_complete_classDeclaration() ||
-            _complete_functionDeclaration() ||
-            _complete_variableDeclaration()) {
+            _complete_variableDeclaration() ||
+            _complete_simpleSemicolon() ||
+            _complete_functionDeclaration()) {
           return completion;
         }
       }

pkg/analysis_server/test/services/completion/statement/statement_completion_test.dart

@@ -1365,6 +1365,23 @@ main() {
 ''');
   }
 
+  test_expressionBody() async {
+    await _prepareCompletion(
+        '=> 1',
+        '''
+class Thing extends Object {
+  int foo() => 1
+}
+''',
+        atEnd: true);
+    _assertHasChange('Add a semicolon and newline', '''
+class Thing extends Object {
+  int foo() => 1;
+  
+}
+''');
+  }
+
   test_noCloseParen() async {
     await _prepareCompletion(
         'ing(3',