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

pkg/compiler/lib/src/js_backend/codegen/codegen.dart

Index: pkg/compiler/lib/src/js_backend/codegen/codegen.dart
diff --git a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
index df8c3070d134f706adc39df77b54fb878edec704..7dced8a008409eaf6b2ed38629ec87217c35e5ff 100644
--- a/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
+++ b/pkg/compiler/lib/src/js_backend/codegen/codegen.dart
@@ -108,6 +108,31 @@ class CodeGenerator extends tree_ir.StatementVisitor
       ++accumulatorIndex;
     }
 
+    // If the last statement is a for loop with an initializer expression, try
+    // to pull that expression into an initializer as well.
+    pullFromForLoop:
+    if (accumulatorIndex < accumulator.length &&
+        accumulator[accumulatorIndex] is js.For) {
+      js.For forLoop = accumulator[accumulatorIndex];
+      if (forLoop.init is! js.Assignment) break pullFromForLoop;
+      js.Assignment assign = forLoop.init;
+      if (assign.leftHandSide is! js.VariableUse) break pullFromForLoop;
+      if (assign.op != null) break pullFromForLoop; // Compound assignment.
+      js.VariableUse use = assign.leftHandSide;
+
+      // We cannot declare a variable more than once.
+      if (!declaredVariables.add(use.name)) break pullFromForLoop;
+
+      js.VariableInitialization jsVariable = new js.VariableInitialization(
+        new js.VariableDeclaration(use.name),
+        assign.value);
+      jsVariables.add(jsVariable);
+
+      // Remove the initializer from the for loop.
+      accumulator[accumulatorIndex] = 
+          new js.For(null, forLoop.condition, forLoop.update, forLoop.body);
+    }
+
     // Discard the statements that were pulled in the initializer.
     if (accumulatorIndex > 0) {
       accumulator = accumulator.sublist(accumulatorIndex);
@@ -516,17 +541,36 @@ class CodeGenerator extends tree_ir.StatementVisitor
     return result;
   }
 
+  js.Expression makeSequence(List<tree_ir.Expression> list) {
+    return list.map(visitExpression).reduce((x,y) => new js.Binary(',', x, y));
+  }
+
   @override
-  void visitWhileCondition(tree_ir.WhileCondition node) {
+  void visitFor(tree_ir.For node) {
     js.Expression condition = visitExpression(node.condition);
     shortBreak.push(node.next);
     shortContinue.push(node);
     fallthrough.push(node);
-    js.Statement jsBody = buildBodyStatement(node.body);
+    js.Statement body = buildBodyStatement(node.body);
     fallthrough.pop();
     shortContinue.pop();
     shortBreak.pop();
-    accumulator.add(insertLabel(node.label, new js.While(condition, jsBody)));
+    js.Statement loopNode;
+    if (node.updates.isEmpty) {
+      loopNode = new js.While(condition, body);
+    } else { // Compile as a for loop.
+      js.Expression init;
+      if (accumulator.isNotEmpty && 
+          accumulator.last is js.ExpressionStatement) {
+        // Take the preceding expression from the accumulator and use
+        // it as the initializer expression.
+        js.ExpressionStatement initStmt = accumulator.removeLast();
+        init = initStmt.expression;
+      }
+      js.Expression update = makeSequence(node.updates);
+      loopNode = new js.For(init, condition, update, body);
+    }
+    accumulator.add(insertLabel(node.label, loopNode));
     visitStatement(node.next);
   }