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 afb3acb83be8268496931bacc242db080cb1f5d7..4e479fe190bd1dbba5a3a50399a3b6ad6e598f4d 100644
--- a/lib/compiler/implementation/lib/js_helper.dart
+++ b/lib/compiler/implementation/lib/js_helper.dart
@@ -811,7 +811,7 @@ unwrapException(ex) {
         // Examples:
         //  x.foo is not a function
         //  'undefined' is not a function (evaluating 'x.foo(1,2,3)')
-        // Object doesn't support property or method 'foo' which sets the error 
+        // Object doesn't support property or method 'foo' which sets the error
         // code 438 in IE.
         // TODO(kasperl): Compute the right name if possible.
         return new NoSuchMethodException('', '<unknown>', []);
@@ -1188,6 +1188,14 @@ interface JavaScriptIndexingBehavior {
 }
 
 /**
+ * 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.
  */