Accept 'null' as document.contentScript.

pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart

Index: pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart
diff --git a/pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart b/pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart
index 3423d30833aba3d5304e043936ee89858e8fd112..f391ed903190fd1536a18838df1015620b4f2b3d 100644
--- a/pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart
+++ b/pkg/compiler/lib/src/js_emitter/main_call_stub_generator.dart
@@ -52,7 +52,9 @@ class MainCallStubGenerator {
           callback(null);
           return;
         }
-        if (document.currentScript) {
+        // When running as a content-script of a chrome-extension the
+        // 'currentScript' is `null` (but not undefined).
+        if (typeof document.currentScript != 'undefined') {
           callback(document.currentScript);
           return;
         }