Future flattening at runtime.

tool/input_sdk/private/ddc_runtime/classes.dart

Index: tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/tool/input_sdk/private/ddc_runtime/classes.dart b/tool/input_sdk/private/ddc_runtime/classes.dart
index f6cb1af603420b0268f107db6021f6bda12df2ce..637a2302ff1b01a491f038a6b79fb87e2f59dafe 100644
--- a/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -75,6 +75,24 @@ final _typeArguments = JS('', 'Symbol("typeArguments")');
 
 final _originalDeclaration = JS('', 'Symbol("originalDeclaration")');
 
+/// Wrap a generic class builder function with future flattening.
+flattenFutures(builder) => JS('', '''(() => {
+  function flatten(T) {
+    if (!T) return $builder($dynamic);
+    let futureClass = $getGenericClass($Future);
+    //TODO(leafp): This only handles the direct flattening case.
+    // It would probably be good to at least search up the class
+    // hierarchy.  If we keep doing flattening long term, we may
+    // want to implement the full future flattening per spec.
+    if ($getGenericClass(T) == futureClass) {
+      let args = $getGenericArgs(T);
+      if (args) return $builder(args[0]);
+    }
+    return $builder(T);
+  }
+  return flatten;
+})()''');
+
 /// Memoize a generic type constructor function.
 generic(typeConstructor) => JS('', '''(() => {
   let length = $typeConstructor.length;