support the JS builtin

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index aeec324d59d20405ff936b7272c586e26187449f..ef02bbf709da3b353aafc253a4f6c54b093369dc 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -757,9 +757,12 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
   JS.Block visitBlock(Block node) => new JS.Block(_visitList(node.statements));
 
   @override
-  JS.Expression visitMethodInvocation(MethodInvocation node) {
+  visitMethodInvocation(MethodInvocation node) {
     var target = node.isCascaded ? _cascadeTarget : node.target;
 
+    var result = _emitForeignJS(node);
+    if (result != null) return result;
+
     if (rules.isDynamicCall(node.methodName)) {
       var args = node.argumentList.accept(this);
       if (target != null) {
@@ -787,6 +790,25 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
     return js.call('#(#)', [targetJs, node.argumentList.accept(this)]);
   }
 
+  /// Emits code for the `JS(...)` builtin.
+  _emitForeignJS(MethodInvocation node) {
+    var e = node.methodName.staticElement;
+    if (e is FunctionElement &&
+        e.library.name == '_foreign_helper' &&
+        e.name == 'JS') {
+      var args = node.argumentList.arguments;
+      // arg[0] is static return type, used in `RestrictedStaticTypeAnalyzer`
+      var code = args[1] as StringLiteral;
+
+      var template = js.parseForeignJS(code.stringValue);
+      var result = template.instantiate(_visitList(args.skip(2)));
+      // `throw` is emitted as a statement by `parseForeignJS`.
+      assert(result is JS.Expression || node.parent is ExpressionStatement);
+      return result;
+    }
+    return null;
+  }
+
   @override
   JS.Expression visitFunctionExpressionInvocation(
       FunctionExpressionInvocation node) {