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

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #library('mock_compiler');
 
 #import("dart:uri");
 
 #import("../../../lib/compiler/implementation/elements/elements.dart");
 #import("../../../lib/compiler/implementation/leg.dart");
@@ skipping 21 matching lines @@
   guard$array(x) { return x; }
   guard$num(x) { return x; }
   guard$string(x) { return x; }
   guard$stringOrArray(x) { return x; }
   index(a, index) {}
   indexSet(a, index, value) {}
   setRuntimeTypeInfo(a, b) {}
   getRuntimeTypeInfo(a) {}
   stringTypeCheck(x) {}
   interface JavaScriptIndexingBehavior {}
-  S() {}''';
+  S() {}
+  assert(a){}''';
 
 const String DEFAULT_INTERCEPTORSLIB = @'''
   add$1(receiver, value) {}
   get$length(receiver) {}
   filter(receiver, predicate) {}
   removeLast(receiver) {}
   iterator(receiver) {}
   next(receiver) {}
   hasNext(receiver) {}''';
 
 const String DEFAULT_CORELIB = @'''
   print(var obj) {}
-  assert(x) {}
   interface int extends num {}
   interface double extends num {}
   class bool {}
   class String {}
   class Object {}
   interface num {}
   class Function {}
   class List {}
   class Closure {}
   class Null {}
@@ skipping 11 matching lines @@
                 bool enableTypeAssertions = false])
       : warnings = [], errors = [],
         sourceFiles = new Map<String, SourceFile>(),
         super(enableTypeAssertions: enableTypeAssertions) {
     Uri uri = new Uri.fromComponents(scheme: "source");
     var script = new Script(uri, new MockFile(coreSource));
     coreLibrary = new LibraryElement(script);
     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'));
 
     script = new Script(uri, new MockFile(helperSource));
     jsHelperLibrary = new LibraryElement(script);
     parseScript(helperSource, jsHelperLibrary);
+    assertMethod = jsHelperLibrary.find(buildSourceString('assert'));
 
     script = new Script(uri, new MockFile(interceptorsSource));
     interceptorsLibrary = new LibraryElement(script);
     parseScript(interceptorsSource, interceptorsLibrary);
 
     mainApp = mockLibrary(this, "");
     initializeSpecialClasses();
   }
 
   void reportWarning(Node node, var message) {
@@ skipping 110 matching lines @@
     });
   }
 }
 
 LibraryElement mockLibrary(Compiler compiler, String source) {
   Uri uri = new Uri.fromComponents(scheme: "source");
   var library = new LibraryElement(new Script(uri, new MockFile(source)));
   importLibrary(library, compiler.coreLibrary, compiler);
   return library;
 }