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

tests/compiler/dart2js/mock_compiler.dart

Index: tests/compiler/dart2js/mock_compiler.dart
diff --git a/tests/compiler/dart2js/mock_compiler.dart b/tests/compiler/dart2js/mock_compiler.dart
index d61a4b452f1adf06adc03cf4f99bdbfead190144..4a3b28da31ca86c4a49b3bc95c6312e14241d618 100644
--- a/tests/compiler/dart2js/mock_compiler.dart
+++ b/tests/compiler/dart2js/mock_compiler.dart
@@ -52,7 +52,7 @@ const String DEFAULT_INTERCEPTORSLIB = @'''
 
 const String DEFAULT_CORELIB = @'''
   print(var obj) {}
-  assert(x) {}
+  _assert(x) {}
   interface int extends num {}
   interface double extends num {}
   class bool {}
@@ -84,7 +84,7 @@ class MockCompiler extends Compiler {
     parseScript(coreSource, coreLibrary);
     // We need to set the assert method to avoid calls with a 'null'
     // target being interpreted as a call to assert.
-    assertMethod = coreLibrary.find(buildSourceString('assert'));
+    assertMethod = coreLibrary.find(buildSourceString('_assert'));
 
     script = new Script(uri, new MockFile(helperSource));
     jsHelperLibrary = new LibraryElement(script);