dart2js: Avoid empty else clauses in generators.

pkg/compiler/lib/src/js/rewrite_async.dart

Index: pkg/compiler/lib/src/js/rewrite_async.dart
diff --git a/pkg/compiler/lib/src/js/rewrite_async.dart b/pkg/compiler/lib/src/js/rewrite_async.dart
index 2a9d8bff3080d0e70493a338fd830305793f5e34..1dcf30502b2adab28ffd540e84b03d04f92aad7a 100644
--- a/pkg/compiler/lib/src/js/rewrite_async.dart
+++ b/pkg/compiler/lib/src/js/rewrite_async.dart
@@ -1,4 +1,4 @@
-// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+tra// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file

[sigurdm, 2016/04/26 05:15:05]

This looks like a mistake

 // 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.
 
@@ -967,7 +967,7 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
       bool oldInsideUntranslatedBreakable = insideUntranslatedBreakable;
       insideUntranslatedBreakable = true;
       addStatement(js.js.statement('do {#} while (#)',
-          [translateInBlock(node.body), visitExpression(node.condition)]));
+          [translateToStatement(node.body), visitExpression(node.condition)]));
       insideUntranslatedBreakable = oldInsideUntranslatedBreakable;
       return;
     }
@@ -1013,7 +1013,7 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
       withExpressions([node.init, node.condition, node.update],
           (List<js.Expression> transformed) {
         addStatement(new js.For(transformed[0], transformed[1], transformed[2],
-            translateInBlock(node.body)));
+            translateToStatement(node.body)));
       });
       insideUntranslatedBreakable = oldInsideUntranslated;
       return;
@@ -1064,13 +1064,15 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
   }
 
   // Only used for code where `!shouldTransform(node)`.
-  js.Block translateInBlock(js.Statement node) {
+  js.Block translateToStatement(js.Statement node) {

[sigurdm, 2016/04/26 05:15:04]

I think the return type should be `js.Statement` now.

     assert(!shouldTransform(node));
     List<js.Statement> oldBuffer = currentStatementBuffer;
     currentStatementBuffer = new List();
     List<js.Statement> resultBuffer = currentStatementBuffer;
     visitStatement(node);
     currentStatementBuffer = oldBuffer;
+    if (resultBuffer.length == 1)
+      return resultBuffer.single;

[sigurdm, 2016/04/26 05:15:04]

Either put the return on the same  line as the if, or surround it with braces.

[sigurdm, 2016/04/26 05:15:05]

For the case where `node` is an empty block (can that even happen?), Should you
not also do
```
if (resultBuffer.isEmpty) {
  return new EmptyStatement;
}
```
?

     return new js.Block(resultBuffer);
   }
 
@@ -1078,8 +1080,9 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
   void visitIf(js.If node) {
     if (!shouldTransform(node.then) && !shouldTransform(node.otherwise)) {
       withExpression(node.condition, (js.Expression condition) {
-        addStatement(new js.If(condition, translateInBlock(node.then),
-            translateInBlock(node.otherwise)));
+        js.Ast translatedThen = translateToStatement(node.then);
+        js.Ast translatedElse = translateToStatement(node.otherwise);
+        addStatement(new js.If(condition, translatedThen, translatedElse));
       }, store: false);
       return;
     }
@@ -1137,7 +1140,7 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
   void visitLabeledStatement(js.LabeledStatement node) {
     if (!shouldTransform(node)) {
       addStatement(
-          new js.LabeledStatement(node.label, translateInBlock(node.body)));
+          new js.LabeledStatement(node.label, translateToStatement(node.body)));
       return;
     }
     // `continue label` is really continuing the nested loop.
@@ -1302,9 +1305,9 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
         List<js.SwitchClause> cases = node.cases.map((js.SwitchClause clause) {
           if (clause is js.Case) {
             return new js.Case(
-                clause.expression, translateInBlock(clause.body));
+                clause.expression, translateToStatement(clause.body));
           } else if (clause is js.Default) {
-            return new js.Default(translateInBlock(clause.body));
+            return new js.Default(translateToStatement(clause.body));
           }
         }).toList();
         addStatement(new js.Switch(key, cases));
@@ -1426,14 +1429,14 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
   /// See the comments of [rewriteFunction] for more explanation.
   void visitTry(js.Try node) {
     if (!shouldTransform(node)) {
-      js.Block body = translateInBlock(node.body);
+      js.Block body = translateToStatement(node.body);

[sigurdm, 2016/04/26 05:15:04]

This would also become a `js.Statement`

       js.Catch catchPart = (node.catchPart == null)
           ? null
           : new js.Catch(node.catchPart.declaration,
-              translateInBlock(node.catchPart.body));
+              translateToStatement(node.catchPart.body));
       js.Block finallyPart = (node.finallyPart == null)
           ? null
-          : translateInBlock(node.finallyPart);
+          : translateToStatement(node.finallyPart);
       addStatement(new js.Try(body, catchPart, finallyPart));
       return;
     }
@@ -1576,7 +1579,7 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
       bool oldInsideUntranslated = insideUntranslatedBreakable;
       insideUntranslatedBreakable = true;
       withExpression(node.condition, (js.Expression condition) {
-        addStatement(new js.While(condition, translateInBlock(node.body)));
+        addStatement(new js.While(condition, translateToStatement(node.body)));
       }, store: false);
       insideUntranslatedBreakable = oldInsideUntranslated;
       return;