dart2js cps: Compile some loops as 'for' loops.

pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart

Index: pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
index 8f0e85666c0f69773ef1daf68001d334d27bb93c..1113cccd360e21f765dd769f57a63c6bdb3e3d32 100644
--- a/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
+++ b/pkg/compiler/lib/src/tree_ir/tree_ir_nodes.dart
@@ -496,8 +496,10 @@ class WhileTrue extends Loop {
 }
 
 /**
- * A while loop with a condition. If the condition is false, control resumes
- * at the [next] statement.
+ * A loop with a condition and update expressions. If there are any update
+ * expressions, this generates a for loop, otherwise a while loop.
+ * 
+ * When the condition is false, control resumes at the [next] statement.
  *
  * It is NOT valid to target this statement with a [Break].
  * The only way to reach [next] is for the condition to evaluate to false.
@@ -508,10 +510,14 @@ class WhileTrue extends Loop {
 class WhileCondition extends Loop {

[Kevin Millikin (Google), 2015/08/13 11:32:47]

Strange that While has updates.  Maybe we call it For.

[asgerf, 2015/08/13 12:06:42]

Renamed.

   final Label label;
   Expression condition;
+  List<Expression> updates;
   Statement body;
   Statement next;
 
-  WhileCondition(this.label, this.condition, this.body,
+  WhileCondition(this.label,
+                 this.condition,
+                 this.updates,
+                 this.body,
                  this.next) {
     assert(label.binding == null);
     label.binding = this;
@@ -1142,6 +1148,7 @@ abstract class RecursiveVisitor implements StatementVisitor, ExpressionVisitor {
 
   visitWhileCondition(WhileCondition node) {
     visitExpression(node.condition);
+    node.updates.forEach(visitExpression);
     visitStatement(node.body);
     visitStatement(node.next);
   }
@@ -1373,6 +1380,7 @@ class RecursiveTransformer extends Transformer {
 
   visitWhileCondition(WhileCondition node) {
     node.condition = visitExpression(node.condition);
+    _replaceExpressions(node.updates);
     node.body = visitStatement(node.body);
     node.next = visitStatement(node.next);
     return node;