A completely new implementation of Es6RewriteGenerators. It aims to g…

…enerate correct code with less boilerplate and supporting a wider range of features.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=187091342

src/com/google/javascript/jscomp/EarlyEs6ToEs3Converter.java

@@ -89,12 +89,6 @@ public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
         // but we want the runtime functions to be have TypeI applied to it by the type checker.
         Es6ToEs3Util.preloadEs6RuntimeFunction(compiler, "makeIterator");
         break;
-      case YIELD:
-        if (n.isYieldAll()) {
-          Es6ToEs3Util.preloadEs6RuntimeFunction(compiler, "makeIterator");
-        }
-        Es6ToEs3Util.preloadEs6Symbol(compiler);
-        break;
       case GETTER_DEF:
       case SETTER_DEF:
         if (compiler.getOptions().getLanguageOut() == LanguageMode.ECMASCRIPT3) {
@@ -107,6 +101,9 @@ public boolean shouldTraverse(NodeTraversal t, Node n, Node parent) {
         if (n.isAsyncFunction()) {
           throw new IllegalStateException("async functions should have already been converted");
         }
+        if (n.isGeneratorFunction()) {
+          compiler.ensureLibraryInjected("es6/generator_engine", /* force= */ false);
+        }
         break;
       default:
         break;
@@ -137,11 +134,6 @@ public void visit(NodeTraversal t, Node n, Node parent) {
           }
         }
         break;
-      case FUNCTION:
-        if (n.isGeneratorFunction()) {
-          Es6RewriteGenerators.preloadGeneratorSkeletonAndReportChange(compiler);
-        }
-        break;
       default:
         break;
     }