Fix check for throwing NSM implementations.

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

Index: pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
diff --git a/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart b/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
index 96e609bb7f8d54113a50f4cda6dfcb7a30e7a01a..70a7936667e8107b4616390e1ce9b0275a20c86f 100644
--- a/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
+++ b/pkg/compiler/lib/src/js_backend/no_such_method_registry.dart
@@ -171,8 +171,9 @@ class NoSuchMethodRegistry {
     } else if (body is Block &&
         !body.statements.isEmpty &&
         body.statements.nodes.tail.isEmpty) {
-      if (body.statements.nodes.head is Throw) {
-        return true;
+      if (body.statements.nodes.head is ExpressionStatement) {
+        ExpressionStatement stmt = body.statements.nodes.head;
+        return stmt.expression is Throw;
       }
     }
     return false;