Reapply "Allow LetCont to bind multiple continuations."

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

 // Copyright (c) 2014, 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 tree_ir_builder;
 
 import '../dart2jslib.dart' as dart2js;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../cps_ir/cps_ir_nodes.dart' as cps_ir;
@@ skipping 341 matching lines @@
     } else {
       return new Assign(variable, definition, visit(node.body));
     }
   }
 
   Statement visitRunnableBody(cps_ir.RunnableBody node) {
     return visit(node.body);
   }
 
   Statement visitLetCont(cps_ir.LetCont node) {
-    Label label;
-    if (node.continuation.hasMultipleUses) {
-      label = new Label();
-      labels[node.continuation] = label;
+    // Introduce labels for continuations that need them.
+    for (cps_ir.Continuation continuation in node.continuations) {
+      if (continuation.hasMultipleUses) {
+        labels[continuation] = new Label();
+      }
     }
     Statement body = visit(node.body);
-    // The continuation's body is not always translated directly here because
-    // it may have been already translated:
+    // Continuations are bound at the same level, but they have to be
+    // translated as if nested.  This is because the body can invoke any
+    // of them from anywhere, so it must be nested inside all of them.
+    //
+    // The continuation bodies are not always translated directly here because
+    // they may have been already translated:
     //   * For singly-used continuations, the continuation's body is
     //     translated at the site of the continuation invocation.
     //   * For recursive continuations, there is a single non-recursive
     //     invocation.  The continuation's body is translated at the site
     //     of the non-recursive continuation invocation.
     // See visitInvokeContinuation for the implementation.
-    if (label == null || node.continuation.isRecursive) return body;
-    return new LabeledStatement(label, body, visit(node.continuation.body));
+    Statement current = body;
+    for (cps_ir.Continuation continuation in node.continuations.reversed) {
+      Label label = labels[continuation];
+      if (label != null && !continuation.isRecursive) {
+        current =
+            new LabeledStatement(label, current, visit(continuation.body));
+      }
+    }
+    return current;
   }
 
   Statement visitInvokeStatic(cps_ir.InvokeStatic node) {
     // Calls are translated to direct style.
     List<Expression> arguments = translateArguments(node.arguments);
     Expression invoke = new InvokeStatic(node.target, node.selector, arguments);
     return continueWithExpression(node.continuation, invoke);
   }
 
   Statement visitInvokeMethod(cps_ir.InvokeMethod node) {
@@ skipping 170 matching lines @@
     // visited.
     internalError(CURRENT_ELEMENT_SPANNABLE, 'Unexpected IR node: $node.');
     return null;
   }
 
   Expression visitIsTrue(cps_ir.IsTrue node) {
     return getVariableReference(node.value);
   }
 }