Don't force for fully split if initializers or updaters do.

lib/src/source_visitor.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 dart_style.src.source_visitor;
 
 import 'package:analyzer/analyzer.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/source.dart';
 
@@ skipping 831 matching lines @@
     builder.startRule();
 
     // The initialization clause.
     if (node.initialization != null) {
       visit(node.initialization);
     } else if (node.variables != null) {
       // Indent split variables more so they aren't at the same level
       // as the rest of the loop clauses.
       builder.indent(Indent.loopVariable);
 
+      // Allow the variables to stay unsplit even if the clauses split.
+      builder.startRule();
+
       var declaration = node.variables;
       visitDeclarationMetadata(declaration.metadata);
       modifier(declaration.keyword);
       visit(declaration.type, after: space);
 
       visitCommaSeparatedNodes(declaration.variables, between: () {
         split();
       });
 
+      builder.endRule();
       builder.unindent();
     }
 
     token(node.leftSeparator);
 
     // The condition clause.
     if (node.condition != null) split();
     visit(node.condition);
     token(node.rightSeparator);
 
     // The update clause.
     if (node.updaters.isNotEmpty) {
       split();
+
+      // Allow the updates to stay unsplit even if the clauses split.
+      builder.startRule();
+
       visitCommaSeparatedNodes(node.updaters, between: split);
+
+      builder.endRule();
     }
 
     token(node.rightParenthesis);
     builder.endRule();
     builder.unnest();
 
     // The body.
     if (node.body is! EmptyStatement) space();
     visit(node.body);
   }
@@ skipping 1251 matching lines @@
   /// Gets the 1-based line number that the beginning of [token] lies on.
   int _startLine(Token token) => _lineInfo.getLocation(token.offset).lineNumber;
 
   /// Gets the 1-based line number that the end of [token] lies on.
   int _endLine(Token token) => _lineInfo.getLocation(token.end).lineNumber;
 
   /// Gets the 1-based column number that the beginning of [token] lies on.
   int _startColumn(Token token) =>
       _lineInfo.getLocation(token.offset).columnNumber;
 }