Do not use full substitution when only computing single type variables.

pkg/compiler/lib/src/js_backend/runtime_types.dart

Index: pkg/compiler/lib/src/js_backend/runtime_types.dart
diff --git a/pkg/compiler/lib/src/js_backend/runtime_types.dart b/pkg/compiler/lib/src/js_backend/runtime_types.dart
index 9aa327d3c95b1bd48eddd7d89a1858a5f373351d..f8aa7b15c6a930dd527ffd4505ed05a4db4772ae 100644
--- a/pkg/compiler/lib/src/js_backend/runtime_types.dart
+++ b/pkg/compiler/lib/src/js_backend/runtime_types.dart
@@ -898,6 +898,27 @@ class Substitution {
       }
     }
   }
+
+  jsAst.Expression getCodeForVariable(int index, RuntimeTypes rti) {
+    jsAst.Expression declaration(TypeVariableType variable) {
+      return new jsAst.Parameter(
+          rti.backend.namer.safeVariableName(variable.name));
+    }
+
+    jsAst.Expression use(TypeVariableType variable) {
+      return new jsAst.VariableUse(
+          rti.backend.namer.safeVariableName(variable.name));
+    }
+
+    if (arguments[index].isDynamic) {
+      return rti.backend.emitter.emitter.generateFunctionThatReturnsNull();
+    } else {
+      jsAst.Expression value =
+          rti.getTypeRepresentation(arguments[index], use);
+      Iterable<jsAst.Expression> formals = parameters.map(declaration);
+      return js('function(#) { return # }', [formals, value]);
+    }
+  }
 }
 
 /**