Separately format lines when a multisplit is split.

lib/src/chunk.dart

Index: lib/src/chunk.dart
diff --git a/lib/src/chunk.dart b/lib/src/chunk.dart
index 58f88af10aba468cbfbca8da3d72f4ea4ad8a83f..e0604275ae4c86cbdeb5ddc628005afd4a624187 100644
--- a/lib/src/chunk.dart
+++ b/lib/src/chunk.dart
@@ -283,7 +283,8 @@ class SplitParam {
 /// together, like parameter lists and binary operator expressions.
 class Span {
   /// Index of the first chunk contained in this span.
-  final int start;
+  int get start => _start;
+  int _start;
 
   /// Index of the last chunk contained in this span.
   int get end => _end;
@@ -293,7 +294,7 @@ class Span {
   /// if the span is for a multisplit.
   final int cost;
 
-  Span(this.start, this.cost);
+  Span(this._start, this.cost);
 
   /// Marks this span as ending at [end].
   void close(int end) {
@@ -314,6 +315,23 @@ class Span {
 
     return result + ")";
   }
+
+  /// Shifts the indexes of the chunk down by [offset].
+  ///
+  /// This is used when a prefix of the chunk list gets pulled off by the
+  /// [LineWriter] after it gets formatted as a line. The remaining spans need
+  /// to have their indices shifted to account for the removed chunks.
+  ///
+  /// Returns `true` if the span has shifted all the way off the front and
+  /// should just be discarded.
+  bool shift(int offset) {
+    if (end != null && end < offset) return true;
+
+    _start -= offset;
+    if (_end != null) _end -= offset;
+
+    return false;
+  }
 }
 
 /// A comment in the source, with a bit of information about the surrounding