Move null check out of getExtensionType

pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart

Index: pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
diff --git a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
index 057e3b875582459b7db9d64b943c8359db93b980..fdaffe8312549fa9a602d278f2355638f2df4e7e 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/ddc_runtime/classes.dart
@@ -214,6 +214,7 @@ getType(obj) =>
     JS('', '# == null ? # : #.__proto__.constructor', obj, Object, obj);
 
 bool isJsInterop(obj) {
+  if (obj == null) return false;
   if (JS('bool', 'typeof # === "function"', obj)) {
     // A function is a Dart function if it has runtime type information.
     return _getRuntimeType(obj) == null;
@@ -391,8 +392,8 @@ bool hasField(type, name) => _hasSigEntry(type, _fieldSig, name);
 
 final _extensionType = JS('', 'Symbol("extensionType")');
 
-getExtensionType(obj) =>
-    JS('', '# == null ? null : #[#]', obj, obj, _extensionType);
+/// This assumes that obj is not null
+getExtensionType(obj) => JS('', '#[#]', obj, _extensionType);
 
 final dartx = JS('', 'dartx');