dart2js allocation tweaks

pkg/compiler/lib/src/io/source_map_builder.dart

 // Copyright (c) 2012, 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 dart2js.source_map_builder;
 
 import 'package:kernel/ast.dart' show Location;
 import '../../compiler_new.dart' show OutputSink, OutputType;
 import '../util/uri_extras.dart' show relativize;
 import '../util/util.dart';
@@ skipping 259 matching lines @@
 class SourceMapEntry {
   SourceLocation sourceLocation;
   int targetOffset;
 
   SourceMapEntry(this.sourceLocation, this.targetOffset);
 }
 
 /// Map from line/column pairs to lists of [T] elements.
 class LineColumnMap<T> {
   Map<int, Map<int, List<T>>> _map = <int, Map<int, List<T>>>{};
+  var _makeLineMap = () => <int, List<T>>{};
+  var _makeList = () => <T>[];
 
   /// Returns the list of elements associated with ([line],[column]).
   List<T> _getList(int line, int column) {
-    Map<int, List<T>> lineMap = _map.putIfAbsent(line, () => <int, List<T>>{});
-    return lineMap.putIfAbsent(column, () => <T>[]);
+    Map<int, List<T>> lineMap = _map.putIfAbsent(line, _makeLineMap);
+    return lineMap.putIfAbsent(column, _makeList);
   }
 
   /// Adds [element] to the end of the list of elements associated with
   /// ([line],[column]).
   void add(int line, int column, T element) {
     _getList(line, column).add(element);
   }
 
   /// Adds [element] to the beginning of the list of elements associated with
   /// ([line],[column]).
@@ skipping 61 matching lines @@
   int register(T element) {
     return map.putIfAbsent(element, () => map.length);
   }
 
   /// Returns the index of [element].
   int operator [](T element) => map[element];
 
   /// Returns the indexed elements.
   Iterable<T> get elements => map.keys;
 }