diff --git a/pkg/analysis_server/test/tool/lsp_spec/typescript_test.dart b/pkg/analysis_server/test/tool/lsp_spec/typescript_test.dart
index 45a7fc1e8d78..fef2206d8b6b 100644
--- a/pkg/analysis_server/test/tool/lsp_spec/typescript_test.dart
+++ b/pkg/analysis_server/test/tool/lsp_spec/typescript_test.dart
@@ -242,6 +242,39 @@ export type DocumentSelector = DocumentFilter[];
       expect(typeAlias.baseType, isArrayOf(isSimpleType('DocumentFilter')));
     });
 
+    test('parses a type alias that is a union of unnamed types', () {
+      final input = '''
+export type NameOrLength = { name: string } | { length: number };
+    ''';
+      final output = parseString(input);
+      expect(output, hasLength(3));
+
+      // Results should be the two inline interfaces followed by the type alias.
+
+      expect(output[0], const TypeMatcher<InlineInterface>());
+      final InlineInterface interface1 = output[0];
+      expect(interface1.name, equals('NameOrLength1'));
+      expect(interface1.members, hasLength(1));
+      expect(interface1.members[0].name, equals('name'));
+
+      expect(output[1], const TypeMatcher<InlineInterface>());
+      final InlineInterface interface2 = output[1];
+      expect(interface2.name, equals('NameOrLength2'));
+      expect(interface2.members, hasLength(1));
+      expect(interface2.members[0].name, equals('length'));
+
+      expect(output[2], const TypeMatcher<TypeAlias>());
+      final TypeAlias typeAlias = output[2];
+      expect(typeAlias.name, equals('NameOrLength'));
+      expect(typeAlias.baseType, const TypeMatcher<UnionType>());
+
+      // The type alias should be a union of the two types above.
+      UnionType union = typeAlias.baseType;
+      expect(union.types, hasLength(2));
+      expect(union.types[0], isSimpleType(interface1.name));
+      expect(union.types[1], isSimpleType(interface2.name));
+    });
+
     test('parses a namespace of constants', () {
       final input = '''
 export namespace ResourceOperationKind {
@@ -321,5 +354,18 @@ interface SomeInformation {
       expect(field.name, equals('label'));
       expect(field.type, isSimpleType('object'));
     });
+
+    test('parses multiple single-line comments into a single token', () {
+      final input = '''
+// This is line 1
+// This is line 2
+interface SomeInformation {
+}
+    ''';
+      final output = parseString(input);
+      expect(output, hasLength(1));
+      expect(output[0].commentNode.token.lexeme, equals('''// This is line 1
+// This is line 2'''));
+    });
   });
 }
diff --git a/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart b/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
index 6fd16917ef2f..f33ab22afed4 100644
--- a/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
+++ b/pkg/analysis_server/tool/lsp_spec/codegen_dart.dart
@@ -36,7 +36,7 @@ bool enumClassAllowsAnyValue(String name) {
 
 String generateDartForTypes(List<AstNode> types) {
   final buffer = IndentableStringBuffer();
-  _getSorted(types).forEach((t) => _writeType(buffer, t));
+  _getSortedUnique(types).forEach((t) => _writeType(buffer, t));
   final formattedCode = _formatCode(buffer.toString());
   return formattedCode.trim() + '\n'; // Ensure a single trailing newline.
 }
@@ -95,9 +95,27 @@ List<Field> _getAllFields(Interface interface) {
       .toList();
 }
 
-/// Returns a copy of the list sorted by name.
-List<AstNode> _getSorted(List<AstNode> items) {
-  final sortedList = items.toList();
+/// Returns a copy of the list sorted by name with duplicates (by name+type) removed.
+List<AstNode> _getSortedUnique(List<AstNode> items) {
+  final uniqueByName = <String, AstNode>{};
+  items.forEach((item) {
+    // It's fine to have the same name used for different types (eg. namespace +
+    // type alias) but some types are just duplicated entirely in the spec in
+    // different positions which should not be emitted twice.
+    final nameTypeKey = '${item.name}|${item.runtimeType}';
+    if (uniqueByName.containsKey(nameTypeKey)) {
+      // At the time of writing, there were two duplicated types:
+      // - TextDocumentSyncKind (same defintion in both places)
+      // - TextDocumentSyncOptions (first definition is just a subset)
+      // If this list grows, consider handling this better - or try to have the
+      // spec updated to be unambigious.
+      print('WARN: More than one definition for $nameTypeKey.');
+    }
+
+    // Keep the last one as in some cases the first definition is less specific.
+    uniqueByName[nameTypeKey] = item;
+  });
+  final sortedList = uniqueByName.values.toList();
   sortedList.sort((item1, item2) => item1.name.compareTo(item2.name));
   return sortedList;
 }
@@ -199,7 +217,7 @@ void _writeCanParseMethod(IndentableStringBuffer buffer, Interface interface) {
     }
     buffer.write('!(');
     _writeTypeCheckCondition(
-        buffer, "obj['${field.name}']", field.type, 'reporter');
+        buffer, interface, "obj['${field.name}']", field.type, 'reporter');
     buffer
       ..write(')) {')
       ..indent()
@@ -302,7 +320,7 @@ void _writeEnumClass(IndentableStringBuffer buffer, Namespace namespace) {
     ..indent();
   if (allowsAnyValue) {
     buffer.writeIndentedln('return ');
-    _writeTypeCheckCondition(buffer, 'obj', typeOfValues, 'reporter');
+    _writeTypeCheckCondition(buffer, null, 'obj', typeOfValues, 'reporter');
     buffer.writeln(';');
   } else {
     buffer
@@ -445,7 +463,8 @@ void _writeFromJsonCodeForUnion(
 
     // Dynamic matches all type checks, so only emit it if required.
     if (!isDynamic) {
-      _writeTypeCheckCondition(buffer, valueCode, type, 'nullLspJsonReporter');
+      _writeTypeCheckCondition(
+          buffer, null, valueCode, type, 'nullLspJsonReporter');
       buffer.write(' ? ');
     }
 
@@ -609,7 +628,7 @@ void _writeMember(IndentableStringBuffer buffer, Member member) {
 }
 
 void _writeMembers(IndentableStringBuffer buffer, List<Member> members) {
-  _getSorted(members).forEach((m) => _writeMember(buffer, m));
+  _getSortedUnique(members).forEach((m) => _writeMember(buffer, m));
 }
 
 void _writeToJsonFieldsForResponseMessage(
@@ -685,8 +704,8 @@ void _writeType(IndentableStringBuffer buffer, AstNode type) {
   }
 }
 
-void _writeTypeCheckCondition(IndentableStringBuffer buffer, String valueCode,
-    TypeBase type, String reporter) {
+void _writeTypeCheckCondition(IndentableStringBuffer buffer,
+    Interface interface, String valueCode, TypeBase type, String reporter) {
   type = resolveTypeAlias(type);
 
   final dartType = type.dartType;
@@ -703,7 +722,8 @@ void _writeTypeCheckCondition(IndentableStringBuffer buffer, String valueCode,
       // TODO(dantup): If we're happy to assume we never have two lists in a union
       // we could skip this bit.
       buffer.write(' && ($valueCode.every((item) => ');
-      _writeTypeCheckCondition(buffer, 'item', type.elementType, reporter);
+      _writeTypeCheckCondition(
+          buffer, interface, 'item', type.elementType, reporter);
       buffer.write('))');
     }
     buffer.write(')');
@@ -711,9 +731,11 @@ void _writeTypeCheckCondition(IndentableStringBuffer buffer, String valueCode,
     buffer.write('($valueCode is Map');
     if (fullDartType != 'dynamic') {
       buffer..write(' && (')..write('$valueCode.keys.every((item) => ');
-      _writeTypeCheckCondition(buffer, 'item', type.indexType, reporter);
+      _writeTypeCheckCondition(
+          buffer, interface, 'item', type.indexType, reporter);
       buffer..write('&& $valueCode.values.every((item) => ');
-      _writeTypeCheckCondition(buffer, 'item', type.valueType, reporter);
+      _writeTypeCheckCondition(
+          buffer, interface, 'item', type.valueType, reporter);
       buffer.write(')))');
     }
     buffer.write(')');
@@ -724,9 +746,18 @@ void _writeTypeCheckCondition(IndentableStringBuffer buffer, String valueCode,
       if (i != 0) {
         buffer.write(' || ');
       }
-      _writeTypeCheckCondition(buffer, valueCode, type.types[i], reporter);
+      _writeTypeCheckCondition(
+          buffer, interface, valueCode, type.types[i], reporter);
     }
     buffer.write(')');
+  } else if (interface != null &&
+      interface.typeArgs != null &&
+      interface.typeArgs.any((typeArg) => typeArg.lexeme == fullDartType)) {
+    final comment = '/* $fullDartType.canParse($valueCode) */';
+    print(
+        'WARN: Unable to write a type check for $valueCode with generic type $fullDartType. '
+        'Please review the generated code annotated with $comment');
+    buffer.write('true $comment');
   } else {
     throw 'Unable to type check $valueCode against $fullDartType';
   }
diff --git a/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart b/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart
index d3038866d1bc..8121590a9ac2 100644
--- a/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart
+++ b/pkg/analysis_server/tool/lsp_spec/typescript_parser.dart
@@ -151,6 +151,7 @@ class Interface extends AstNode {
     this.baseTypes,
     this.members,
   ) : super(comment);
+
   @override
   String get name => nameToken.lexeme;
   String get nameWithTypeArgs => '$name$typeArgsString';
@@ -502,7 +503,9 @@ class Parser {
     if (includeUndefined) {
       types.add(Type.Undefined);
     }
+    var typeIndex = 0;
     while (true) {
+      typeIndex++;
       TypeBase type;
       if (_match([TokenType.LEFT_BRACE])) {
         // Inline interfaces.
@@ -521,7 +524,12 @@ class Parser {
           type = MapType(indexer.indexType, indexer.valueType);
         } else {
           // Add a synthetic interface to the parsers list of nodes to represent this type.
-          final generatedName = _joinNames(containerName, fieldName);
+          // If we have no fieldName to base the synthetic name from, we should use
+          // the index of this type, for example in:
+          //    type Foo = { [..] } | { [...] }
+          // we will generate Foo1 and Foo2 for the types.
+          final nameSuffix = fieldName ?? '$typeIndex';
+          final generatedName = _joinNames(containerName, nameSuffix);
           _nodes.add(InlineInterface(generatedName, members));
           // Record the type as a simple type that references this interface.
           type = Type.identifier(generatedName);
@@ -616,7 +624,9 @@ class Parser {
     final name = _consume(TokenType.IDENTIFIER, 'Expected identifier');
     _consume(TokenType.EQUAL, 'Expected =');
     final type = _type(name.lexeme, null);
-    _consume(TokenType.SEMI_COLON, 'Expected ;');
+    if (!_isAtEnd) {
+      _consume(TokenType.SEMI_COLON, 'Expected ;');
+    }
 
     return TypeAlias(leadingComment, name, type);
   }
@@ -640,8 +650,16 @@ class Scanner {
     return _tokens;
   }
 
-  void _addToken(TokenType type) {
-    final text = _source.substring(_startOfToken, _currentPos);
+  void _addToken(TokenType type, {bool mergeSameTypes = false}) {
+    var text = _source.substring(_startOfToken, _currentPos);
+
+    // Consecutive tokens of some types (for example Comments) are merged
+    // together.
+    if (mergeSameTypes && _tokens.isNotEmpty && type == _tokens.last.type) {
+      text = '${_tokens.last.lexeme}\n$text';
+      _tokens.removeLast();
+    }
+
     _tokens.add(Token(type, text));
   }
 
@@ -744,13 +762,13 @@ class Scanner {
             _advance();
             _advance();
           }
-          _addToken(TokenType.COMMENT);
+          _addToken(TokenType.COMMENT, mergeSameTypes: true);
         } else if (_match('/')) {
           // Single line comment.
           while (_peek() != '\n' && !_isAtEnd) {
             _advance();
           }
-          _addToken(TokenType.COMMENT);
+          _addToken(TokenType.COMMENT, mergeSameTypes: true);
         } else {
           _addToken(TokenType.SLASH);
         }