dart2js: Smaller async error sequence when only one handler

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 b1918ad5ecded07ba04bedb2ededf87029f64c21..06437d7b5bf1b3bd76787daa1b3ae261e5592458 100644
--- a/pkg/compiler/lib/src/js/rewrite_async.dart
+++ b/pkg/compiler/lib/src/js/rewrite_async.dart
@@ -127,6 +127,14 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
   js.VariableUse get handler => new js.VariableUse(handlerName);
   String handlerName;
 
+  /// Set to `true` if any of the switch statement labels is a handler. At the
+  /// end of rewriting this is used to see if a shorter form of error handling
+  /// can be used. The shorter form could be a change in the method boilerplate,
+  /// in the state machine wrapper, or not implemented. [addErrorExit] can test
+  /// this to elide the error exit handler when there are no other handlers, or
+  /// set it to `true` if there is no shorter form.
+  bool hasHandlerLabels = false;
+
   /// A stack of labels of finally blocks to visit, and the label to go to after
   /// the last.
   js.VariableUse get next => new js.VariableUse(nextName);
@@ -685,8 +693,10 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
 
     variables.add(_makeVariableInitializer(goto, js.number(0)));
     variables.addAll(variableInitializations());
-    variables.add(_makeVariableInitializer(handler, js.number(rethrowLabel)));
-    variables.add(_makeVariableInitializer(currentError, null));
+    if (hasHandlerLabels) {
+      variables.add(_makeVariableInitializer(handler, js.number(rethrowLabel)));
+      variables.add(_makeVariableInitializer(currentError, null));
+    }
     if (analysis.hasFinally || (isAsyncStar && analysis.hasYield)) {
       variables.add(_makeVariableInitializer(
           next, new js.ArrayInitializer(<js.Expression>[])));
@@ -1412,6 +1422,7 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
   }
 
   setErrorHandler([int errorHandler]) {
+    hasHandlerLabels = true; // TODO(sra): Add short form error handler.
     js.Expression label =
         (errorHandler == null) ? currentErrorHandler : js.number(errorHandler);
     addStatement(js.js.statement('# = #;', [handler, label]));
@@ -1708,6 +1719,7 @@ class AsyncRewriter extends AsyncRewriterBase {
   }
 
   void addErrorExit() {
+    if (!hasHandlerLabels) return; // rethrow handled in method boilerplate.
     beginLabel(rethrowLabel);
     addStatement(js.js.statement(
         "return #thenHelper(#currentError, #completer);", {
@@ -1777,8 +1789,11 @@ class AsyncRewriter extends AsyncRewriterBase {
           #variableDeclarations;
           var #bodyName = #wrapBody(function (#errorCode, #result) {
             if (#errorCode === #ERROR) {
-                #currentError = #result;
-                #goto = #handler;
+              if (#hasHandlerLabels) {
+                  #currentError = #result;
+                  #goto = #handler;
+              } else
+                  return #asyncRethrow(#result, #completer);
             }
             #rewrittenBody;
           });
@@ -1796,6 +1811,8 @@ class AsyncRewriter extends AsyncRewriterBase {
           "errorCode": errorCodeName,
           "result": resultName,
           "asyncStart": asyncStart,
+          "asyncRethrow": asyncRethrow,
+          "hasHandlerLabels": hasHandlerLabels,
           "completer": completer,
           "wrapBody": wrapBody,
         }).withSourceInformation(sourceInformation);
@@ -1907,6 +1924,7 @@ class SyncStarRewriter extends AsyncRewriterBase {
   }
 
   void addErrorExit() {
+    hasHandlerLabels = true; // TODO(sra): Add short form error handler.
     beginLabel(rethrowLabel);
     addStatement(js.js
         .statement('return #(#);', [uncaughtErrorExpression, currentError]));
@@ -2091,6 +2109,7 @@ class AsyncStarRewriter extends AsyncRewriterBase {
 
   @override
   void addErrorExit() {
+    hasHandlerLabels = true;
     beginLabel(rethrowLabel);
     addStatement(js.js.statement(
         "return #asyncHelper(#currentError, #errorCode, #controller);", {