Prepare templates for short circuiting async functions.

Today:
async function f() {
  var x = 10;
  return x;
}

Is transpiled to:
function f() {
  return $jscomp.executeAsyncGenerator(
      function $jscomp$async$generator() {
        var x;
        return $jscomp.generator.createGenerator(
            $jscomp$async$generator,
            function($jscomp$generator$context) {
              x = 10;
              return $jscomp$generator$context.return(x);
            });
      });
};

Will be transpiled to:
function f() {
  var x;
  return $jscomp.generator.asyncExecutePromiseGenerator(
      function ($jscomp$generator$context) {
        x = 10;
        return $jscomp$generator$context.return(x);
      });
};

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=193372804

src/com/google/javascript/jscomp/js/es6/execute_async_generator.js

@@ -16,6 +16,7 @@
 
 'require base';
 'require es6/promise';
+'require es6/generator_engine';
 
 /**
  * Handles the execution of an async function.
@@ -98,3 +99,19 @@ $jscomp.asyncExecutePromiseGenerator = function(generator) {
 $jscomp.asyncExecutePromiseGeneratorFunction = function(generatorFunction) {
   return $jscomp.asyncExecutePromiseGenerator(generatorFunction());
 };
+
+/**
+ * Handles the execution of a state machine program that represents transpiled
+ * async function.
+ *
+ * @final
+ * @param {function(!$jscomp.generator.Context<?>): (void|{value: ?})} program
+ * @return {!Promise<?>}
+ * @suppress {reportUnknownTypes, visibility}
+ */
+$jscomp.asyncExecutePromiseGeneratorProgram = function(program) {
+  return $jscomp.asyncExecutePromiseGenerator(
+      new $jscomp.generator.Generator_(
+          new $jscomp.generator.Engine_(
+              program)));
+};