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

lib/compiler/implementation/lib/js_helper.dart

Index: lib/compiler/implementation/lib/js_helper.dart
diff --git a/lib/compiler/implementation/lib/js_helper.dart b/lib/compiler/implementation/lib/js_helper.dart
index 683929aa218674aa657813331af055b9885bbc21..cfe19d18aecc0e25dca8dae0a7b2fd3548e36e70 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -1222,6 +1222,14 @@ class FallThroughErrorImplementation implements FallThroughError {
 }
 
 /**
+ * Helper function for implementing asserts. The compiler treats this specially.
+ */
+void assert(condition) {
+  if (condition is Function) condition = condition();
+  if (!condition) throw new AssertionError();
+}
+
+/**
  * Called by generated code when a method that must be statically
  * resolved cannot be found.
  */