Support general handling of type variables by substituting types into the current context.

sdk/lib/_internal/compiler/implementation/dart2jslib.dart

 // Copyright (c) 2012, 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.
 
 library dart2js;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 
 import 'closure.dart' as closureMapping;
 import 'dart_backend/dart_backend.dart' as dart_backend;
 import 'dart_types.dart';
 import 'elements/elements.dart';
 import 'elements/modelx.dart'
     show ErroneousElementX,
          ClassElementX,
          CompilationUnitElementX,
          LibraryElementX,
          PrefixElementX,
          VoidElementX,
          AnalyzableElement,
          DeferredLoaderGetterElementX,
          SynthesizedCallMethodElementX;
+import 'helpers/helpers.dart';
 import 'js_backend/js_backend.dart' as js_backend;
 import 'native_handler.dart' as native;
 import 'scanner/scannerlib.dart';
 import 'ssa/ssa.dart';
 import 'tree/tree.dart';
 import 'ir/ir_builder.dart' show IrBuilderTask;
 import 'universe/universe.dart';
 import 'util/util.dart';
 import 'util/characters.dart' show $_;
 import '../compiler.dart' as api;
@@ skipping 29 matching lines @@
 part 'constant_system_dart.dart';
 part 'diagnostic_listener.dart';
 part 'enqueue.dart';
 part 'library_loader.dart';
 part 'resolved_visitor.dart';
 part 'script.dart';
 part 'tree_validator.dart';
 part 'typechecker.dart';
 part 'warnings.dart';
 part 'world.dart';