Set experimentalDecorators warning for JavaScript

(cherry picked from commit ed7abcc)

Author: billti

src/compiler/checker.ts

@@ -12827,7 +12827,7 @@ namespace ts {
             }
 
             if (!compilerOptions.experimentalDecorators) {
-                error(node, Diagnostics.Experimental_support_for_decorators_is_a_feature_that_is_subject_to_change_in_a_future_release_Specify_experimentalDecorators_to_remove_this_warning);
+                error(node, Diagnostics.Experimental_support_for_decorators_is_a_feature_that_is_subject_to_change_in_a_future_release_Set_experimentalDecorators_to_remove_this_warning);
             }
 
             if (compilerOptions.emitDecoratorMetadata) {

src/compiler/diagnosticMessages.json

@@ -679,7 +679,7 @@
         "category": "Error",
         "code": 1218
     },
-    "Experimental support for decorators is a feature that is subject to change in a future release. Specify '--experimentalDecorators' to remove this warning.": {
+    "Experimental support for decorators is a feature that is subject to change in a future release. Set 'experimentalDecorators' to remove this warning.": {
         "category": "Error",
         "code": 1219
     },

src/compiler/program.ts

@@ -827,6 +827,11 @@ namespace ts {
                             let typeAssertionExpression = <TypeAssertion>node;
                             diagnostics.push(createDiagnosticForNode(typeAssertionExpression.type, Diagnostics.type_assertion_expressions_can_only_be_used_in_a_ts_file));
                             return true;
+                        case SyntaxKind.Decorator:
+                            if (!options.experimentalDecorators) {
+                                diagnostics.push(createDiagnosticForNode(node, Diagnostics.Experimental_support_for_decorators_is_a_feature_that_is_subject_to_change_in_a_future_release_Set_experimentalDecorators_to_remove_this_warning))
+                            }
+                            return true;
                     }
 
                     return forEachChild(node, walk);