Implement substitution for type variables.

sdk/lib/_internal/compiler/implementation/js_backend/namer.dart

Index: sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
diff --git a/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart b/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
index e5bb9670d04277801fc55c50dd91eb8b898374b1..fa6f91e0a59498dc43e58a6202955993b3772208 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
@@ -669,6 +669,8 @@ class Namer implements ClosureNamer {
 
   String operatorIsPrefix() => r'$is';
 
+  String operatorAsPrefix() => r'$as';
+
   String operatorIs(Element element) {
     // TODO(erikcorry): Reduce from $isx to ix when we are minifying.
     return '${operatorIsPrefix()}${getName(element)}';
@@ -686,6 +688,10 @@ class Namer implements ClosureNamer {
     return name;
   }
 
+  String substitutionName(Element element) {
+    return '${operatorAsPrefix()}${getName(element)}';
+  }
+
   String safeName(String name) => _safeName(name, jsReserved);
   String safeVariableName(String name) => _safeName(name, jsVariableReserved);