Change assert implementation to not depend on a top-level function called 'assert'.

lib/compiler/implementation/elements/elements.dart

Index: lib/compiler/implementation/elements/elements.dart
diff --git a/lib/compiler/implementation/elements/elements.dart b/lib/compiler/implementation/elements/elements.dart
index 58ae435cc5e9c51a95f877703dc2b81c78e83ed2..a0d294d8ea57a8ecdcc50fa6e9849355c8383b87 100644
--- a/lib/compiler/implementation/elements/elements.dart
+++ b/lib/compiler/implementation/elements/elements.dart
@@ -158,6 +158,7 @@ class Element implements Hashable {
   bool isTypedef() => kind === ElementKind.TYPEDEF;
   bool isTypeVariable() => kind === ElementKind.TYPE_VARIABLE;
   bool isField() => kind === ElementKind.FIELD;
+  bool isAbstractField() => kind === ElementKind.ABSTRACT_FIELD;
   bool isGetter() => kind === ElementKind.GETTER;
   bool isSetter() => kind === ElementKind.SETTER;
   bool isAccessor() => isGetter() || isSetter();