Avoid boxing loop variables that are only mutated in initializer or update

pkg/compiler/lib/src/closure.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 closureToClassMapper;
 
 import "elements/elements.dart";
 import "dart2jslib.dart";
 import "dart_types.dart";
 import "js_backend/js_backend.dart" show JavaScriptBackend;
@@ skipping 800 matching lines @@
     scopeVariables = oldScopeVariables;
   }
 
   visitLoop(Loop node) {
     inNewScope(node, () {
       node.visitChildren(this);
     });
   }
 
   visitFor(For node) {
-    visitLoop(node);
+    inNewScope(node, () {
+      // First visit initializer and update so we can easily check if a loop
+      // variable was captured in one of these subexpressions.
+      if (node.initializer != null) visit(node.initializer);
+      if (node.update != null) visit(node.update);
+
+      // Loop variables that have not been captured yet can safely be flagged as
+      // non-mutated, because no nested function can observe the mutation.
+      if (node.initializer is VariableDefinitions) {
+        VariableDefinitions definitions = node.initializer;
+        definitions.definitions.nodes.forEach((Node node) {
+          LocalVariableElement local = elements[node];
+          if (!isCapturedVariable(local)) {
+            mutatedVariables.remove(local);
+          }
+        });
+      }
+
+      // Visit condition and body.
+      // This must happen after the above, so any loop variables mutated in the
+      // condition or body are indeed flagged as mutated.
+      if (node.conditionStatement != null) visit(node.conditionStatement);
+      if (node.body != null) visit(node.body);
+    });
     // See if we have declared loop variables that need to be boxed.
     if (node.initializer == null) return;
     VariableDefinitions definitions = node.initializer.asVariableDefinitions();
     if (definitions == null) return;
     ClosureScope scopeData = closureData.capturingScopes[node];
     if (scopeData == null) return;
     List<LocalVariableElement> result = <LocalVariableElement>[];
     for (Link<Node> link = definitions.definitions.nodes;
          !link.isEmpty;
          link = link.tail) {
@@ skipping 160 matching lines @@
 
   String get name => typeVariable.name;
 
   int get hashCode => typeVariable.hashCode;
 
   bool operator ==(other) {
     if (other is! TypeVariableLocal) return false;
     return typeVariable == other.typeVariable;
   }
 }