Use "self" instead of globalThis.

dart/sdk/lib/_internal/lib/preambles/jsshell.js

Index: dart/sdk/lib/_internal/lib/preambles/jsshell.js
diff --git a/dart/sdk/lib/_internal/lib/preambles/jsshell.js b/dart/sdk/lib/_internal/lib/preambles/jsshell.js
index f24555fd54405ea0b535d771bf16d1a6cff9703b..e3b4f1aae35b8331be29ad66d2fa1648432758a2 100644
--- a/dart/sdk/lib/_internal/lib/preambles/jsshell.js
+++ b/dart/sdk/lib/_internal/lib/preambles/jsshell.js
@@ -3,3 +3,12 @@
 // BSD-style license that can be found in the LICENSE file.
 
 // Javascript preamble, that lets the output of dart2js run on JSShell.
+
+(function(self) {
+  // Using strict mode to avoid accidentally defining global variables.
+  "use strict"; // Should be first statement of this function.
+
+  // Global properties. "self" refers to the global object, so adding a
+  // property to "self" defines a global variable.
+  self.self = self;
+})(this)