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 6a5067dba110a0c14eaf3df0eaf5388e9f9be5a3..69cdd8e0469265b2a2251ee1a3d6be702741f526 100644
--- a/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
+++ b/sdk/lib/_internal/compiler/implementation/js_backend/namer.dart
@@ -523,6 +523,8 @@ 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)}';
@@ -540,6 +542,10 @@ class Namer implements ClosureNamer {
     return name;
   }
 
+  String substitutionName(Element element) {
+    return '${operatorAsPrefix()}${getName(element)}';
+  }
+
   String oneShotInterceptorName(Selector selector) {
     // TODO(ngeoffray): What to do about typed selectors? We could
     // filter them out, or keep them and hope the generated one shot