fix #27971, implement generic function RTTI

pkg/dev_compiler/lib/src/compiler/code_generator.dart

Index: pkg/dev_compiler/lib/src/compiler/code_generator.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/code_generator.dart b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
index 35553f6059607735ebe14b8ae7b023444b25d6d2..5866870795b4c6b7889adaf99ac693c3d05011d2 100644
--- a/pkg/dev_compiler/lib/src/compiler/code_generator.dart
+++ b/pkg/dev_compiler/lib/src/compiler/code_generator.dart
@@ -750,18 +750,34 @@ class CodeGenerator extends Object
   }
 
   @override
-  visitFunctionTypeAlias(FunctionTypeAlias node) {
-    FunctionTypeAliasElement element = node.element;
+  visitFunctionTypeAlias(FunctionTypeAlias node) => _emitTypedef(node);
+
+  @override
+  visitGenericTypeAlias(GenericTypeAlias node) => _emitTypedef(node);
+
+  JS.Statement _emitTypedef(TypeAlias node) {
+    var element = node.element as FunctionTypeAliasElement;
+    FunctionType type;
+    var typeFormals = element.typeParameters;
+    if (element is GenericTypeAliasElement) {
+      type = element.function.type;
+    } else {
+      type = element.type;
+      if (typeFormals.isNotEmpty) {
+        // Skip past the type formals, we'll add them back below, so these
+        // type parameter names will end up in scope in the generated JS.
+        type = type.instantiate(typeFormals.map((f) => f.type).toList());
+      }
+    }
 
     JS.Expression body = annotate(
         _callHelper('typedef(#, () => #)', [
           js.string(element.name, "'"),
-          _emitType(element.type, nameType: false, lowerTypedef: true)
+          _emitType(type, nameType: false, lowerTypedef: true)
         ]),
         node,
         element);
 
-    var typeFormals = element.typeParameters;
     if (typeFormals.isNotEmpty) {
       return _defineClassTypeArguments(element, typeFormals,
           js.statement('const # = #;', [element.name, body]));
@@ -771,12 +787,6 @@ class CodeGenerator extends Object
   }
 
   @override
-  visitGenericTypeAlias(GenericTypeAlias node) {
-    throw new UnimplementedError('Generic type aliases are not implemented. '
-        'See https://github.com/dart-lang/sdk/issues/27971');
-  }
-
-  @override
   JS.Expression visitTypeName(TypeName node) {
     if (node.type == null) {
       // TODO(jmesserly): if the type fails to resolve, should we generate code
@@ -3037,7 +3047,7 @@ class CodeGenerator extends Object
     }
 
     var typeFormals = type.typeFormals;
-    if (typeFormals.isNotEmpty && !lowerTypedef) {
+    if (typeFormals.isNotEmpty) {
       // TODO(jmesserly): this is a suboptimal representation for universal
       // function types (as callable functions). See discussion at
       // https://github.com/dart-lang/sdk/issues/27333