Null check remover understands JS fragments.

pkg/compiler/lib/src/js/js.dart

Index: pkg/compiler/lib/src/js/js.dart
diff --git a/pkg/compiler/lib/src/js/js.dart b/pkg/compiler/lib/src/js/js.dart
index 1a08cfbeb0798072ea178fd3993b134183c5c718..3a9fd2ea8e599045266b3d5a1060191852a1c0b8 100644
--- a/pkg/compiler/lib/src/js/js.dart
+++ b/pkg/compiler/lib/src/js/js.dart
@@ -162,4 +162,27 @@ class UnparsedNode extends DeferredString
 
   @override
   String get value => _literal.value;
-}
+}
+
+/// Returns `true` if [template] will immediately give a TypeError if the first
+/// placeholder is `null` or `undefined`.
+bool isNullGuardOnFirstArgument(Template template) {
+  // We look for a template of the form
+  //
+  //     #.something
+  //     #.something()
+  //
+  Node node = template.ast;
+  if (node is Call) {
+    Call call = node;
+    node = call.target;
+  }
+  if (node is PropertyAccess) {
+    PropertyAccess access = node;
+    if (access.receiver is InterpolatedExpression) {
+      InterpolatedExpression hole = access.receiver;
+      return hole.isPositional && hole.nameOrPosition == 0;
+    }
+  }
+  return false;
+}