JS: prevent generation of excessive this.$result statements in coroutines

Author: Alexey Andreev

js/js.translator/src/org/jetbrains/kotlin/js/translate/context/TemporaryVariable.java

@@ -45,7 +45,9 @@ protected TemporaryVariable(@NotNull JsName temporaryName, @Nullable JsExpressio
 
     @NotNull
     public JsNameRef reference() {
-        return variableName.makeRef();
+        JsNameRef result = variableName.makeRef();
+        MetadataProperties.setSynthetic(result, true);
+        return result;
     }
 
     @NotNull

js/js.translator/src/org/jetbrains/kotlin/js/translate/utils/JsAstUtils.java

@@ -53,8 +53,9 @@ public static JsStatement convertToStatement(@NotNull JsNode jsNode) {
         assert (jsNode instanceof JsExpression) || (jsNode instanceof JsStatement)
                 : "Unexpected node of type: " + jsNode.getClass().toString();
         if (jsNode instanceof JsExpression) {
-            JsExpressionStatement statement = new JsExpressionStatement((JsExpression) jsNode);
-            if (jsNode instanceof JsNullLiteral) {
+            JsExpression expression = (JsExpression) jsNode;
+            JsExpressionStatement statement = new JsExpressionStatement(expression);
+            if (expression instanceof JsNullLiteral || MetadataProperties.getSynthetic(expression)) {
                 MetadataProperties.setSynthetic(statement, true);
             }
             return statement;