Implement [Generator].prototype.return.

src/js/generator.js

Index: src/js/generator.js
diff --git a/src/js/generator.js b/src/js/generator.js
index 7f43656ebcf5c1fb4cfb5eca311e8381082f5c33..6aad8e1624e7abe9b9fa687d497b2e25e1ce87dc 100644
--- a/src/js/generator.js
+++ b/src/js/generator.js
@@ -37,14 +37,44 @@ function GeneratorObjectNext(value) {
     // Generator is suspended.
     DEBUG_PREPARE_STEP_IN_IF_STEPPING(this);
     try {
-      return %_GeneratorNext(this, value);
+      let result = %_GeneratorNext(this, value);
+      if (result.done) %GeneratorClose(this);
+      return result;
     } catch (e) {
       %GeneratorClose(this);
       throw e;
     }
   } else if (continuation == 0) {
     // Generator is already closed.
-    return { value: void 0, done: true };
+    return %_CreateIterResultObject(UNDEFINED, true);
+  } else {
+    // Generator is running.
+    throw MakeTypeError(kGeneratorRunning);
+  }
+}
+
+
+function GeneratorObjectReturn(value) {
+  if (!IS_GENERATOR(this)) {
+    throw MakeTypeError(kIncompatibleMethodReceiver,
+                        '[Generator].prototype.return', this);
+  }
+
+  var continuation = %GeneratorGetContinuation(this);
+  if (continuation > 0) {
+    // Generator is suspended.
+    DEBUG_PREPARE_STEP_IN_IF_STEPPING(this);
+    try {
+      let result = %_GeneratorReturn(this, value);
+      if (result.done) %GeneratorClose(this);
+      return result;
+    } catch (e) {
+      %GeneratorClose(this);
+      throw e;
+    }
+  } else if (continuation == 0) {
+    // Generator is already closed.
+    return %_CreateIterResultObject(value, true);
   } else {
     // Generator is running.
     throw MakeTypeError(kGeneratorRunning);
@@ -62,7 +92,9 @@ function GeneratorObjectThrow(exn) {
   if (continuation > 0) {
     // Generator is suspended.
     try {
-      return %_GeneratorThrow(this, exn);
+      let result = %_GeneratorThrow(this, exn);
+      if (result.done) %GeneratorClose(this);
+      return result;
     } catch (e) {
       %GeneratorClose(this);
       throw e;
@@ -78,9 +110,11 @@ function GeneratorObjectThrow(exn) {
 
 // ----------------------------------------------------------------------------
 
-// Both Runtime_GeneratorNext and Runtime_GeneratorThrow are supported by
-// neither Crankshaft nor TurboFan, disable optimization of wrappers here.
+// None of the three resume operations (Runtime_GeneratorNext,
+// Runtime_GeneratorReturn, Runtime_GeneratorThrow) is supported by
+// Crankshaft or TurboFan.  Disable optimization of wrappers here.
 %NeverOptimizeFunction(GeneratorObjectNext);
+%NeverOptimizeFunction(GeneratorObjectReturn);
 %NeverOptimizeFunction(GeneratorObjectThrow);
 
 // Set up non-enumerable functions on the generator prototype object.
@@ -88,6 +122,7 @@ var GeneratorObjectPrototype = GeneratorFunctionPrototype.prototype;
 utils.InstallFunctions(GeneratorObjectPrototype,
                        DONT_ENUM,
                       ["next", GeneratorObjectNext,
+                       "return", GeneratorObjectReturn,
                        "throw", GeneratorObjectThrow]);
 
 %AddNamedProperty(GeneratorObjectPrototype, "constructor",