dart:mirrors workaround for generic methods

Fixes #29509

R=jmesserly@google.com

Review-Url: https://codereview.chromium.org/2852723002 .

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/types.dart

@@ -512,6 +512,19 @@ functionType(returnType, args, extra) =>
 definiteFunctionType(returnType, args, extra) =>
     _functionType(true, returnType, args, extra);
 
+///
+/// TODO(vsm): Remove when mirrors is deprecated.
+/// This is a temporary workaround to support dart:mirrors, which doesn't
+/// understand generic methods.
+///
+getFunctionTypeMirror(AbstractFunctionType type) {
+  if (type is GenericFunctionType) {
+    var typeArgs = new List.filled(type.formalCount, dynamic);
+    return type.instantiate(typeArgs);
+  }
+  return type;
+}
+
 bool isType(obj) => JS(
     '',
     '''(() => {

pkg/dev_compiler/tool/input_sdk/private/js_mirrors.dart

@@ -81,6 +81,10 @@ dynamic _getMixins(type) {
   return JS('', '#.getMixins(#, [])', _dart, type);
 }
 
+dynamic _getFunctionType(type) {
+  return JS('', '#.getFunctionTypeMirror(#)', _dart, type);
+}
+
 typedef T _Lazy<T>();
 
 dynamic _getESSymbol(Symbol symbol) =>
@@ -627,11 +631,7 @@ class JsMethodMirror extends JsMirror implements MethodMirror {
 
     // TODO(vsm): Handle generic function types properly.  Or deprecate mirrors
     // before we need to!
-    if (JS('bool', 'typeof(#) == "function"', ftype)) {
-      // Instantiate the generic version.
-      // TODO(vsm): Can't use arguments.length on arrow function.
-      ftype = JS('', '#.apply(null, #)', ftype, [dynamic, dynamic, dynamic]);
-    }
+    ftype = _getFunctionType(ftype);
 
     // TODO(vsm): Add named args.
     List args = ftype.args;

tests/lib_strong/mirrors/generic_method_test.dart

@@ -0,0 +1,15 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:mirrors';
+import 'package:expect/expect.dart';
+
+class Foo {
+  T bar<T>() => null;
+}
+
+void main() {
+  var type = reflectClass(Foo);
+  Expect.isTrue(type.declarations.keys.contains(#bar));
+}