Allow `return <expr>` in sync* and async* javascript.

tests/compiler/dart2js/async_await_js_transform_test.dart

 // Copyright (c) 2015, 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.
 
 import "package:expect/expect.dart";
 import "package:compiler/src/js/js.dart";
 import "package:compiler/src/js/rewrite_async.dart";
 import "package:compiler/src/js_backend/js_backend.dart" show StringBackedName;
 
 import "backend_dart/dart_printer_test.dart" show PrintDiagnosticListener;
 
-void testTransform(String source, String expected) {
+void testTransform(String source, String expected, AsyncRewriterBase rewriter) {
   Fun fun = js(source);
-  Fun rewritten = new AsyncRewriter(
-      null, // The diagnostic helper should not be used in these tests.
-      null,
-      asyncHelper: new VariableUse("thenHelper"),
-      newCompleter: new VariableUse("Completer"),
-      safeVariableName: (String name) => "__$name",
-      bodyName: new StringBackedName("body")).rewrite(fun);
+  Fun rewritten = rewriter.rewrite(fun);
 
   JavaScriptPrintingOptions options = new JavaScriptPrintingOptions();
   SimpleJavaScriptPrintingContext context =
       new SimpleJavaScriptPrintingContext();
   Printer printer = new Printer(options, context);
   printer.visit(rewritten);
   Expect.stringEquals(expected, context.getText());
 }
 
+void testAsyncTransform(String source, String expected) {
+  testTransform(source, expected, new AsyncRewriter(
+      null, // The diagnostic helper should not be used in these tests.
+      null,
+      asyncHelper: new VariableUse("thenHelper"),
+      newCompleter: new VariableUse("Completer"),
+      safeVariableName: (String name) => "__$name",
+      bodyName: new StringBackedName("body")));
+}
+
+void testSyncStarTransform(String source, String expected) {
+  testTransform(source, expected, new SyncStarRewriter(
+      null,
+      null,
+      endOfIteration: new VariableUse("endOfIteration"),
+      newIterable: new VariableUse("newIterable"),
+      yieldStarExpression: new VariableUse("yieldStar"),
+      uncaughtErrorExpression: new VariableUse("uncaughtError"),
+      safeVariableName: (String name) => "__$name",
+      bodyName: new StringBackedName("body")));
+}
+
 main() {
-  testTransform("""
+  testAsyncTransform("""
 function(a) async {
   print(this.x); // Ensure `this` is translated in the helper function.
   await foo();
 }""", """
 function(a) {
   var __goto = 0, __completer = new Completer(), __handler = 1, __currentError, __self = this;
   function body(__errorCode, __result) {
     if (__errorCode === 1) {
       __currentError = __result;
       __goto = __handler;
@@ skipping 10 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-  testTransform("""
+  testAsyncTransform("""
   function(b) async {
     try {
       __outer: while (true) { // Overlapping label name.
         try {
           inner: while (true) {
             break __outer; // Break from untranslated loop to translated target.
             break; // break to untranslated target.
           }
           while (true) {
             return; // Return via finallies.
@@ skipping 103 matching lines @@
             // return
             return thenHelper(__returnValue, 0, __completer, null);
           case 2:
             // rethrow
             return thenHelper(__currentError, 1, __completer);
         }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-    testTransform("""
+  testAsyncTransform("""
 function(c) async {
   var a, b, c, d, e, f;
   a = b++; // post- and preincrements.
   b = --b;
   c = (await foo()).a++;
   d = ++(await foo()).a;
   e = foo1()[await foo2()]--;
   f = --foo1()[await foo2()];
 }""", """
 function(c) {
@@ skipping 34 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-  testTransform("""
+  testAsyncTransform("""
   function(d2) async {
     var a, b, c, d, e, f, g, h; // empty initializer
     a = foo1() || await foo2(); // short circuiting operators
     b = await foo1() || foo2();
     c = await foo1() || foo3(await foo2());
     d = foo1() || foo2();
     e = foo1() && await foo2();
     f = await foo1() && foo2();
     g = await foo1() && await foo2();
     h = foo1() && foo2();
@@ skipping 112 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-    testTransform("""
+  testAsyncTransform("""
 function(x, y) async {
   while (true) {
     switch(y) { // Switch with no awaits in case key expressions
       case 0:
       case 1: 
         await foo();
         continue; // Continue the loop, not the switch
       case 1: // Duplicate case
         await foo();
         break; // Break the switch
@@ skipping 73 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-  testTransform("""
+  testAsyncTransform("""
   function(f) async {
     do {
       var a = await foo();
       if (a) // If with no awaits in body
         break;
       else
         continue;
     } while (await foo());
   }
   """, """
@@ skipping 40 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-    testTransform("""
+  testAsyncTransform("""
 function(g) async {
   for (var i = 0; i < await foo1(); i += await foo2()) {
     if (foo(i))
       continue;
     else
       break;
     if (!foo(i)) { // If with no else and await in body.
       await foo();
       return;
     }
@@ skipping 69 matching lines @@
           // return
           return thenHelper(__returnValue, 0, __completer, null);
         case 2:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-  testTransform("""
+  testAsyncTransform("""
   function(a, h) async {
     var x = {"a": foo1(), "b": await foo2(), "c": foo3()};
     x["a"] = 2; // Different assignments
     (await foo()).a = 3;
     x[await foo()] = 4;
     x[(await foo1()).a = await foo2()] = 5;
     (await foo1())[await foo2()] = await foo3(6);
   }
   """, """
 function(a, h) {
@@ skipping 54 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-    testTransform("""
+  testAsyncTransform("""
 function(c, i) async {
   try {
     var x = c ? await foo() : foo(); // conditional
     var y = {};
   } catch (error) {
     try {
       x = c ? await fooError(error) : fooError(error);
     } catch (error) { // nested error handler with overlapping name
       y.x = foo(error);
     } finally {
@@ skipping 96 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-  testTransform("""
+  testAsyncTransform("""
   function(x, y, j) async {
     print(await(foo(x))); // calls
     (await print)(foo(x));
     print(foo(await x));
     await (print(foo(await x)));
     print(foo(x, await y, z));
   }
   """, """
 function(x, y, j) {
   var __goto = 0, __completer = new Completer(), __handler = 1, __currentError, __temp1, __temp2, __temp3;
@@ skipping 45 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-    testTransform("""
+  testAsyncTransform("""
 function(x, y, k) async {
   while (await(foo())) {
     lab: { // labelled statement
       switch(y) {
       case 0:
         foo();
       case 0: // Duplicate case
         print(await foo1(x));
         return y;
       case await bar(): // await in case
@@ skipping 118 matching lines @@
         case 1:
           // return
           return thenHelper(__returnValue, 0, __completer, null);
         case 2:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
-  testTransform("""
+
+  testAsyncTransform("""
   function(l) async {
     switch(await l) {
       case 1:
         print(1);
         break;
       case 2:
         print(1);
         // Fallthrough
       default:
         print(2);
@@ skipping 28 matching lines @@
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
 
-  testTransform("""
+  testAsyncTransform("""
   function(m) async {
     var exception = 1;
     try {
       await 42;
       throw 42;
     } catch (exception) {
       exception = await 10;
       exception += await 10;
       exception++;
       exception--;
@@ skipping 57 matching lines @@
           print(exception);
           // implicit return
           return thenHelper(null, 0, __completer, null);
         case 1:
           // rethrow
           return thenHelper(__currentError, 1, __completer);
       }
   }
   return thenHelper(null, body, __completer, null);
 }""");
+
+  testSyncStarTransform("""
+function(a) sync* {
+  // Ensure that return of a value is treated as first evaluating the value, and
+  // then returning.
+  return foo();
+}""", """
+function(__a) {
+  return new newIterable(function() {
+    var a = __a;
+    var __goto = 0, __handler = 2, __currentError;
+    return function body(__errorCode, __result) {
+      if (__errorCode === 1) {
+        __currentError = __result;
+        __goto = __handler;
+      }
+      while (true)
+        switch (__goto) {
+          case 0:
+            // Function start
+            foo();
+            // goto return
+            __goto = 1;
+            break;
+          case 1:
+            // return
+            return endOfIteration();
+          case 2:
+            // rethrow
+            return uncaughtError(__currentError);
+        }
+    };
+  });
+}""");
 }