Better handling for binary operators in => bodies.

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 167 matching lines @@
   }
 
   visitAwaitExpression(AwaitExpression node) {
     token(node.awaitKeyword);
     space();
     visit(node.expression);
   }
 
   visitBinaryExpression(BinaryExpression node) {
     builder.startSpan();
-    builder.nestExpression();
+
+    // If a binary operator sequence appears immediately after a `=>`, don't
+    // add an extra level of nesting. Instead, let the subsequent operands line
+    // up with the first, as in:
+    //
+    //     method() =>
+    //         argument &&
+    //         argument &&
+    //         argument;
+    var isArrowBody = node.parent is ExpressionFunctionBody;
+    if (!isArrowBody) builder.nestExpression();
 
     // Start lazily so we don't force the operator to split if a line comment
     // appears before the first operand.
     builder.startLazyRule();
 
     // Flatten out a tree/chain of the same precedence. If we split on this
     // precedence level, we will break all of them.
     var precedence = node.operator.type.precedence;
 
     traverse(Expression e) {
@@ skipping 11 matching lines @@
     }
 
     // Blocks as operands to infix operators should always nest like regular
     // operands. (Granted, this case is exceedingly rare in real code.)
     builder.startBlockArgumentNesting();
 
     traverse(node);
 
     builder.endBlockArgumentNesting();
 
-    builder.unnest();
+    if (!isArrowBody) builder.unnest();
     builder.endSpan();
     builder.endRule();
   }
 
   visitBlock(Block node) {
     // Don't allow splitting in an empty block.
     if (node.statements.isEmpty &&
         node.rightBracket.precedingComments == null) {
       token(node.leftBracket);
       token(node.rightBracket);
@@ skipping 471 matching lines @@
 
     // Try to keep the "(...) => " with the start of the body for anonymous
     // functions.
     if (_isInLambda(node)) builder.startSpan();
 
     token(node.functionDefinition); // "=>".
 
     // Split after the "=>", using the rule created before the parameters
     // by _visitBody().
     split();
-    builder.endRule();
+
+    // If the body is a binary operator expression, then we want to force the
+    // split at `=>` if the operators split. See visitBinaryExpression().
+    if (node.expression is! BinaryExpression) builder.endRule();
 
     if (_isInLambda(node)) builder.endSpan();
 
     builder.startBlockArgumentNesting();
     builder.startSpan();
     visit(node.expression);
     builder.endSpan();
     builder.endBlockArgumentNesting();
 
+    if (node.expression is BinaryExpression) builder.endRule();
+
     token(node.semicolon);
   }
 
   visitExpressionStatement(ExpressionStatement node) {
     _simpleStatement(node, () {
       visit(node.expression);
     });
   }
 
   visitExtendsClause(ExtendsClause node) {
@@ skipping 1608 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;
 }